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/quickfix.c b/src/quickfix.c
index c533a28..4e1e8e0 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -6263,7 +6263,7 @@
 	if (!did_bufnr_emsg)
 	{
 	    did_bufnr_emsg = TRUE;
-	    semsg(_("E92: Buffer %ld not found"), bufnum);
+	    semsg(_("E92: Buffer %d not found"), bufnum);
 	}
 	valid = FALSE;
 	bufnum = 0;