Xext: fix missing include of <X11/Xmd.h>

Including <X11/extensions/securproto.h> depends on <X11/Xmd.h> has been included before.

Bug: #1719 (closed) Fixes: b30edf32 - fix missing includes of <X11/Xfuncproto.h> Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net