patch 8.1.1304: MS-Windows: compiler warning for unused value

Problem:    MS-Windows: compiler warning for unused value.
Solution:   Adjust #ifdefs. (Ken Takata, closes #4363)
diff --git a/src/gui.c b/src/gui.c
index 471202b..da7284f 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -69,7 +69,7 @@
 {
     char_u	*old_term;
     static int	recursive = 0;
-#ifdef GUI_MAY_SPAWN
+#if defined(GUI_MAY_SPAWN) && defined(EXPERIMENTAL_GUI_CMD)
     char	*msg = NULL;
 #endif
 
@@ -113,7 +113,10 @@
 # endif
 	    )
     {
-	msg = gui_mch_do_spawn(arg);
+# ifdef EXPERIMENTAL_GUI_CMD
+	msg =
+# endif
+	    gui_mch_do_spawn(arg);
     }
     else
 #endif