Fix compiler warning when printing uint64_t

lib/tcp_rigctl_msg_source_impl.cc

@@ -17,6 +17,8 @@
  *  You should have received a copy of the GNU General Public License
  *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
+#define __STDC_FORMAT_MACROS
+#include <inttypes.h>
 
 #ifdef HAVE_CONFIG_H
 #include "config.h"
@@ -86,7 +88,7 @@ namespace gr
     send_freq (int sock, uint64_t freq)
     {
       static char buf[512];
-      snprintf (buf, 512, "%llu\n", freq);
+      snprintf (buf, 512, "%" PRIu64 "\n", freq);
       send (sock, buf, strnlen (buf, 512), 0);
     }