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/if_cscope.c b/src/if_cscope.c
index e1408b2..4904424 100644
--- a/src/if_cscope.c
+++ b/src/if_cscope.c
@@ -651,7 +651,7 @@
 cs_reading_emsg(
     int idx)	/* connection index */
 {
-    semsg(_("E262: error reading cscope connection %ld"), idx);
+    semsg(_("E262: error reading cscope connection %d"), idx);
 }
 
 #define	CSREAD_BUFSIZE	2048