video: Don't compile isAtLeastGL3() if we don't have OpenGL support _at all_.

src/video/SDL_video.c

@@ -2809,11 +2809,13 @@ SDL_GL_UnloadLibrary(void)
     }
 }
 
+#if SDL_VIDEO_OPENGL || SDL_VIDEO_OPENGL_ES || SDL_VIDEO_OPENGL_ES2
 static SDL_INLINE SDL_bool
 isAtLeastGL3(const char *verstr)
 {
     return (verstr && (SDL_atoi(verstr) >= 3));
 }
+#endif
 
 SDL_bool
 SDL_GL_ExtensionSupported(const char *extension)