Fixed building with older macOS SDK

src/video/cocoa/SDL_cocoaclipboard.m

@@ -25,6 +25,10 @@
 #include "SDL_cocoavideo.h"
 #include "../../events/SDL_clipboardevents_c.h"
 
+#if MAC_OS_X_VERSION_MAX_ALLOWED < 101300
+typedef NSString *NSPasteboardType; /* Defined in macOS 10.13+ */
+#endif
+
 @interface Cocoa_PasteboardDataProvider : NSObject<NSPasteboardItemDataProvider>
 {
     SDL_ClipboardDataCallback m_callback;