macosx_application: replace mp_msg with stderr for semi-fatal error

Apparently this should never be run anyway.

osdep/macosx_application.m

@@ -16,6 +16,7 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#include <stdio.h>
 #include <pthread.h>
 #include "talloc.h"
 
@@ -336,7 +337,7 @@ int cocoa_main(mpv_main_fn mpv_main, int argc, char *argv[])
 
         // This should never be reached: cocoa_run_runloop blocks until the
         // process is quit
-        mp_msg(MSGT_CPLAYER, MSGL_ERR, "There was either a problem "
+        fprintf(stderr, "There was either a problem "
                 "initializing Cocoa or the Runloop was stopped unexpectedly. "
                 "Please report this issues to a developer.\n");
         pthread_join(playback_thread_id, NULL);