dix: fix misspelled header guard

Fix FTBS on BSD:

> In file included from ../dix/colormap.c:57:
> ../dix/colormap_priv.h:5:9: warning: '_XSERVER_DIX_COLORMAP_PRIV_H' is used as a header guard here, followed by #define of a different macro [-Wheader-guard]
>         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ../dix/colormap_priv.h:6:9: note: '_XSERVER_DIX_COLORMAP_PRIv_H' is defined here; did you mean '_XSERVER_DIX_COLORMAP_PRIV_H'?
>         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
>         _XSERVER_DIX_COLORMAP_PRIV_H
> 1 warning generated.
> [7/531] Compiling C object dix/liblibxserver_dix.a.p/devices.c.o
> In file included from ../dix/devices.c:62:
> ../dix/ptrveloc_priv.h:20:18: warning: redefinition of typedef 'PointerAccelerationProfileFunc' is a C11 feature [-Wtypedef-redefinition]
> typedef double (*PointerAccelerationProfileFunc)
>                  ^
> ../include/ptrveloc.h:50:18: note: previous definition is here
> typedef double (*PointerAccelerationProfileFunc)
>                  ^
> In file included from ../dix/devices.c:62:
> ../dix/ptrveloc_priv.h:33:3: warning: redefinition of typedef 'MotionTracker' is a C11 feature [-Wtypedef-redefinition]
> } MotionTracker, *MotionTrackerPtr;
>   ^
> ../include/ptrveloc.h:54:31: note: previous definition is here
> typedef struct _MotionTracker MotionTracker, *MotionTrackerPtr;
>                               ^
> In file included from ../dix/devices.c:62:
> ../dix/ptrveloc_priv.h:33:19: warning: redefinition of typedef 'MotionTrackerPtr' is a C11 feature [-Wtypedef-redefinition]
> } MotionTracker, *MotionTrackerPtr;
>                   ^
> ../include/ptrveloc.h:54:47: note: previous definition is here
> typedef struct _MotionTracker MotionTracker, *MotionTrackerPtr;
>                                               ^
> 3 warnings generated.
> [8/531] Compiling C object dix/liblibxserver_dix.a.p/main.c.o
> In file included from ../dix/main.c:86:
> ../dix/callback_priv.h:10:31: warning: redefinition of typedef 'CallbackListPtr' is a C11 feature [-Wtypedef-redefinition]
> typedef struct _CallbackList *CallbackListPtr;
>                               ^
> ../include/callback.h:60:31: note: previous definition is here
> typedef struct _CallbackList *CallbackListPtr;  /* also in misc.h */
>                               ^
> 1 warning generated.
> [9/531] Compiling C object dix/liblibxserver_dix.a.p/dispatch.c.o
> In file included from ../dix/dispatch.c:111:
> ../dix/screenint_priv.h:11:25: warning: redefinition of typedef 'ScreenPtr' is a C11 feature [-Wtypedef-redefinition]
> typedef struct _Screen *ScreenPtr;
>                         ^
> ../include/screenint.h:55:25: note: previous definition is here
> typedef struct _Screen *ScreenPtr;
>                         ^
> 1 warning generated.
> [10/531] Compiling C object dix/liblibxserver_dix.a.p/dixutils.c.o
> In file included from ../dix/dixutils.c:90:
> ../dix/callback_priv.h:10:31: warning: redefinition of typedef 'CallbackListPtr' is a C11 feature [-Wtypedef-redefinition]
> typedef struct _CallbackList *CallbackListPtr;