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/sign.c b/src/sign.c
index 734dd25..6f7ead9 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -162,7 +162,7 @@
 	group = HI2SG(hi);
     }
 
-    // Search for the next usuable sign identifier
+    // Search for the next usable sign identifier
     while (!found)
     {
 	if (group == NULL)
@@ -996,7 +996,7 @@
 
     if ((lnum = buf_findsign(buf, sign_id, sign_group)) <= 0)
     {
-	semsg(_("E157: Invalid sign ID: %ld"), sign_id);
+	semsg(_("E157: Invalid sign ID: %d"), sign_id);
 	return -1;
     }