mac80211: use __printf attribute in debugfs

The internal function mac80211_format_buffer() has a
printf-style argument list, so add the attribute to
have gcc verify that list.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/debugfs.h

@@ -2,8 +2,8 @@
 #define __MAC80211_DEBUGFS_H
 
 #ifdef CONFIG_MAC80211_DEBUGFS
-extern void debugfs_hw_add(struct ieee80211_local *local);
-extern int mac80211_format_buffer(char __user *userbuf, size_t count,
+void debugfs_hw_add(struct ieee80211_local *local);
+int __printf(4, 5) mac80211_format_buffer(char __user *userbuf, size_t count,
 					  loff_t *ppos, char *fmt, ...);
 #else
 static inline void debugfs_hw_add(struct ieee80211_local *local)