ui/cocoa: Declare that QemuCocoaAppController implements NSApplicationDelegate

Our class QemuCocoaAppController implements the NSApplicationDelegate
interface, and we pass an object of this class to [NSApp setDelegate].
However, we weren't declaring in the class definition that we implemented
this interface; in OSX 10.10 this provokes the following (slighly
misleading) warning:
ui/cocoa.m:1031:24: warning: sending 'QemuCocoaAppController *' to parameter of
      incompatible type 'id<NSFileManagerDelegate>'
    [NSApp setDelegate:appController];
                       ^~~~~~~~~~~~~
/System/Library/Frameworks/Foundation.framework/Headers/NSFileManager.h:109:47:
note: passing argument to parameter 'delegate' here
@property (assign) id <NSFileManagerDelegate> delegate NS_AVAILABLE(10_5,
2_0);
                                              ^

Annoyingly, this interface wasn't formally defined until OSX 10.6, so we
have to surround the relevant part of the @interface line with an ifdef.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1431296361-16981-5-git-send-email-peter.maydell@linaro.org

ui/cocoa.m

@@ -784,6 +784,9 @@ QemuCocoaView *cocoaView;
  ------------------------------------------------------
 */
 @interface QemuCocoaAppController : NSObject
+#if (MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_6)
+                                             <NSApplicationDelegate>
+#endif
 {
 }
 - (void)startEmulationWithArgc:(int)argc argv:(char**)argv;