util: Add printf format GCC attribute to panic() function

This fixes the following GCC warning:

  util.h:55:2: warning: function might be possible candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

compiler.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2015 Tobias Klauser <tklauser@distanz.ch>
+ * Copyright (C) 2015-2017 Tobias Klauser <tklauser@distanz.ch>
  *
  * This file is part of llmnrd.
  *
@@ -28,6 +28,10 @@
 # ifndef __unused
 #  define __unused		__attribute__((unused))
 # endif
+# ifndef __check_format_printf
+#  define __check_format_printf(__fmt, __args)	\
+				__attribute__ ((format (printf, (__fmt), (__args))))
+# endif
 # ifndef offsetof
 #  define offsetof(a, b)	__builtin_offsetof(a, b)
 # endif

util.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2014-2015 Tobias Klauser <tklauser@distanz.ch>
+ * Copyright (C) 2014-2017 Tobias Klauser <tklauser@distanz.ch>
  * Copyright (C) 2009-2012 Daniel Borkmann
  *
  * This file is part of llmnrd.
@@ -47,6 +47,8 @@
 	(void) (&_max1 == &_max2);	\
 	_max1 > _max2 ? _max1 : _max2; })
 
+static inline void panic(const char *fmt, ...) __check_format_printf(1, 2);
+
 static inline void __noreturn panic(const char *fmt, ...)
 {
 	va_list vl;