patch 8.0.1677: no compiler warning for wrong format in vim_snprintf()

Problem:    No compiler warning for wrong format in vim_snprintf().
Solution:   Add printf attribute for gcc.  Fix reported problems.
diff --git a/src/spellfile.c b/src/spellfile.c
index e9eacbe..496e07f 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -3649,7 +3649,7 @@
 	{
 	    spin->si_msg_count = 0;
 	    vim_snprintf((char *)message, sizeof(message),
-		    _("line %6d, word %6d - %s"),
+		    _("line %6d, word %6ld - %s"),
 		       lnum, spin->si_foldwcount + spin->si_keepwcount, w);
 	    msg_start();
 	    msg_puts_long_attr(message, 0);