patch 8.1.0791: a few compiler warnings on VMS

Problem:    A few compiler warnings on VMS.
Solution:   Remove type cast.  Adjust #ifdef. (Zoltan Arpadffy)
diff --git a/src/os_unix.c b/src/os_unix.c
index 33ef781..3757dc5 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -1935,7 +1935,7 @@
 	alarm(0);
 	signal(SIGALRM, (RETSIGTYPE (*)())sig_save);
 	if (p_verbose > 0 && sig_alarm_called)
-	    verb_msg((char_u *)_("Opening the X display timed out"));
+	    verb_msg(_("Opening the X display timed out"));
 #endif
 	if (x11_display != NULL)
 	{