patch 8.1.0753: printf format not checked for semsg()

Problem:    printf format not checked for semsg().
Solution:   Add GNUC attribute and fix reported problems. (Dominique Pelle,
            closes #3805)
diff --git a/src/diff.c b/src/diff.c
index 3acedc7..ed63f51 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -173,7 +173,7 @@
 	    return;
 	}
 
-    semsg(_("E96: Cannot diff more than %ld buffers"), DB_COUNT);
+    semsg(_("E96: Cannot diff more than %d buffers"), DB_COUNT);
 }
 
 /*