Added a missing include file to nanovg_gl.h.

To make the examples link correctly under OS X 12.3, you need to include
<OpenGL/glext.h> in the file.  This commit limits what happens by protecting
the include as follows:

    #ifdef __APPLE__
    #   include <OpenGL/glext.h>
    #endif

Thus, this should have no effect on non-Apple systems.

src/nanovg_gl.h

@@ -113,6 +113,10 @@ enum NVGimageFlagsGL {
 #include <math.h>
 #include "nanovg.h"
 
+#ifdef __APPLE__
+#   include <OpenGL/glext.h>
+#endif
+
 enum GLNVGuniformLoc {
 	GLNVG_LOC_VIEWSIZE,
 	GLNVG_LOC_TEX,