log: Check printf() arguments

At present logging does not check printf() arguments. Now that all users
have been corrected, enable this to prevent further problems.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/log.h b/include/log.h
index 0f2bc19..d7f6471 100644
--- a/include/log.h
+++ b/include/log.h
@@ -73,7 +73,8 @@
  * @return 0 if log record was emitted, -ve on error
  */
 int _log(enum log_category_t cat, enum log_level_t level, const char *file,
-	 int line, const char *func, const char *fmt, ...);
+	 int line, const char *func, const char *fmt, ...)
+		__attribute__ ((format (__printf__, 6, 7)));
 
 /* Define this at the top of a file to add a prefix to debug messages */
 #ifndef pr_fmt