patch 9.0.0694: no native sound support on Mac OS

Problem:    No native sound support on Mac OS.
Solution:   Add sound support for Mac OS. (Yee Cheng Chin, closes #11274)
diff --git a/src/os_unix.c b/src/os_unix.c
index 145f93f..814f3ad 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -6125,6 +6125,10 @@
 		rest -= msec;
 	}
 # endif
+# ifdef FEAT_SOUND_MACOSX
+	// Invoke any pending sound callbacks.
+	process_cfrunloop();
+# endif
 # ifdef FEAT_SOUND_CANBERRA
 	// Invoke any pending sound callbacks.
 	if (has_sound_callback_in_queue())