Use GCC_FMT_ATTR (format checking)

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/json-parser.c b/json-parser.c
index 70b9b6f..6c06ef9 100644
--- a/json-parser.c
+++ b/json-parser.c
@@ -91,7 +91,8 @@
 /**
  * Error handler
  */
-static void parse_error(JSONParserContext *ctxt, QObject *token, const char *msg, ...)
+static void GCC_FMT_ATTR(3, 4) parse_error(JSONParserContext *ctxt,
+                                           QObject *token, const char *msg, ...)
 {
     va_list ap;
     va_start(ap, msg);