SDL_cocoamodes.m: fix build against older SDKs.

Fixes https://github.com/libsdl-org/SDL/issues/5208

src/video/cocoa/SDL_cocoamodes.m

@@ -40,6 +40,9 @@
 #ifndef MAC_OS_X_VERSION_10_13
 #define NSAppKitVersionNumber10_12 1504
 #endif
+#if (IOGRAPHICSTYPES_REV < 40)
+#define kDisplayModeNativeFlag 0x02000000
+#endif
 
 
 static void