patch 9.0.0237: Mac: cannot build if dispatch.h is not available

Problem:    Mac: cannot build if dispatch.h is not available.
Solution:   Add #ifdef. (Evan Miller, closes #10954)
diff --git a/src/os_macosx.m b/src/os_macosx.m
index acc5b07..f666b71 100644
--- a/src/os_macosx.m
+++ b/src/os_macosx.m
@@ -28,7 +28,9 @@
 #include <sys/errno.h>
 #include <stdlib.h>
 
+#ifdef FEAT_RELTIME
 #include <dispatch/dispatch.h>
+#endif
 
 #include "vim.h"
 #import <AppKit/AppKit.h>