Gcc and clang can check format strings of printf-like functions

Since we don't support any other compilers right now, enable that
extension unconditionally.

Fix the bugs found by format string checking.

Corresponding to flashrom svn r1032.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Sean Nelson <audiohacked@gmail.com>
diff --git a/flashrom.c b/flashrom.c
index 8a97454..73145a9 100644
--- a/flashrom.c
+++ b/flashrom.c
@@ -394,7 +394,7 @@
 int register_shutdown(void (*function) (void *data), void *data)
 {
 	if (shutdown_fn_count >= SHUTDOWN_MAXFN) {
-		msg_perr("Tried to register more than %n shutdown functions.\n",
+		msg_perr("Tried to register more than %i shutdown functions.\n",
 			 SHUTDOWN_MAXFN);
 		return 1;
 	}