tests/examples/gl/gtk: build failure on Mac OS X

Actually, I found this error while fixing gst-build#13 (closed)

I guess it should be temporarily disabled.

In file included from ../subprojects/gst-plugins-base/tests/examples/gl/gtk/gstgtk.c:37:
In file included from /usr/local/Cellar/gtk+3/3.22.30/include/gtk-3.0/gdk/gdkquartz.h:23:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/AppKit.framework/Headers/AppKit.h:10:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:8:
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:512:1: error: expected identifier or '('
@class NSString, Protocol;
^
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSObjCRuntime.h:514:9: error: unknown type name 'NSString'; did you mean 'GString'?
typedef NSString * NSExceptionName NS_EXTENSIBLE_STRING_ENUM;
        ^
/usr/local/Cellar/glib/2.58.1/include/glib-2.0/glib/gstring.h:39:33: note: 'GString' declared here
typedef struct _GString         GString;
                                ^