AP_HAL_Linux: RCInput_UDP: Fix compiling warning

This warning happens because of the difference of datatypes between
32 and 64 bits processors.

%% libraries/AP_HAL_Linux/RCInput_UDP.o
/home/zehortigoza/dev/ardupilot/libraries/AP_HAL_Linux/RCInput_UDP.cpp: In member function 'virtual void Linux::LinuxRCInput_UDP::_timer_tick()':
/home/zehortigoza/dev/ardupilot/libraries/AP_HAL_Linux/RCInput_UDP.cpp:42:72: warning: format '%llu' expects argument of type 'long long unsigned int', but argument 3 has type 'uint64_t {aka long unsigned int}' [-Wformat=]
             hal.console->printf("no rc cmds received for %llu\n", delay);

libraries/AP_HAL_Linux/RCInput_UDP.cpp

@@ -39,7 +39,7 @@ void LinuxRCInput_UDP::_timer_tick(void)
         }
         if (_last_buf_ts != 0 &&
             (delay = _buf.timestamp_us - _last_buf_ts) > 100000) {
-            hal.console->printf("no rc cmds received for %llu\n", delay);
+            hal.console->printf("no rc cmds received for %llu\n", (unsigned long long)delay);
         }
         _last_buf_ts = _buf.timestamp_us;