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/spellfile.c b/src/spellfile.c
index 3d68409..bf580eb 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -6014,7 +6014,7 @@
     else if (vim_strchr(gettail(wfname), '_') != NULL)
 	emsg(_("E751: Output file name must not have region name"));
     else if (incount > MAXREGIONS)
-	semsg(_("E754: Only up to %ld regions supported"), MAXREGIONS);
+	semsg(_("E754: Only up to %d regions supported"), MAXREGIONS);
     else
     {
 	/* Check for overwriting before doing things that may take a lot of
@@ -6274,7 +6274,7 @@
 		break;
 	    if (*spf == NUL)
 	    {
-		semsg(_("E765: 'spellfile' does not have %ld entries"), idx);
+		semsg(_("E765: 'spellfile' does not have %d entries"), idx);
 		vim_free(fnamebuf);
 		return;
 	    }