mraa.c: fix FTBFS on printf by adding format

gcc-4.8.2 fails to build from source and complains :

  error: format not a string literal and no format arguments

Signed-off-by: Philippe Coval <philippe.coval@open.eurogiciel.org>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/mraa.c

@@ -79,8 +79,8 @@ mraa_init()
         char *err_msg = "mraa: FATAL error, "
                         "libmraa program must be run as root (EUID 0), "
                         "cannot proceed\n";
-        syslog(LOG_ERR, err_msg);
-        fprintf(stderr, err_msg);
+        syslog(LOG_ERR, "%s", err_msg);
+        fprintf(stderr, "%s", err_msg);
         return MRAA_ERROR_PLATFORM_NOT_INITIALISED;
     }