Fix gcc warning

src/hb-private.hh

@@ -620,6 +620,15 @@ _hb_debug (unsigned int level,
 #define DEBUG_LEVEL_ENABLED(WHAT, LEVEL) (_hb_debug ((LEVEL), HB_DEBUG_##WHAT))
 #define DEBUG_ENABLED(WHAT) (DEBUG_LEVEL_ENABLED (WHAT, 0))
 
+template <int max_level> static inline void
+_hb_debug_msg_va (const char *what,
+		  const void *obj,
+		  const char *func,
+		  bool indented,
+		  unsigned int level,
+		  int level_dir,
+		  const char *message,
+		  va_list ap) HB_PRINTF_FUNC(7, 0);
 template <int max_level> static inline void
 _hb_debug_msg_va (const char *what,
 		  const void *obj,