libobs-opengl: Disable deprecation warnings on macOS

We are aware of OpenGL having been deprecated on macOS since
Mac OS X 10.14, so silence the deprecation warnings the "official" way.

libobs-opengl/gl-cocoa.m

@@ -16,6 +16,7 @@
 ******************************************************************************/
 
 #include "gl-subsystem.h"
+#define GL_SILENCE_DEPRECATION
 #include <OpenGL/OpenGL.h>
 
 #import <Cocoa/Cocoa.h>