gdkmacosglcontext: fix compilation error
This fixes the following compilation error:
../gdk/macos/gdkmacosglcontext.c:107:5: error: cast to smaller integer type 'NSOpenGLPixelFormatAttribute' (aka 'unsigned int') from 'void *' [-Werror,-Wvoid-pointer-to-int-cast]
(NSOpenGLPixelFormatAttribute)nil
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.