Remove compiler warning when U_CX_PORT_PRINTF is defined as nothing.

When using uxclient with ubxlib we define U_CX_AT_CONFIG_FILE in order to use our own configuration file. When ubxlib is configured to have no logging at compile-time, so U_CX_PORT_PRINTF is defined to be empty, we get a compiler warning from uCxLogPrintTime() because the stack variables in that function are no longer used. This PR makes the whole of the body of uCxLogPrintTime() conditional on U_CX_PORT_PRINTF being defined in order to remove the warning.

src/u_cx_log.c

@@ -53,13 +53,15 @@ static bool gUCxLogEnabled = true;
 
 void uCxLogPrintTime(void)
 {
+#if U_CX_PORT_PRINTF
     int32_t timestamp_ms = U_CX_PORT_GET_TIME_MS();
     int32_t ms      = (int32_t) (timestamp_ms % 1000);
     int32_t seconds = (int32_t) (timestamp_ms / 1000) % 60 ;
     int32_t minutes = (int32_t) ((timestamp_ms / (1000 * 60)) % 60);
     int32_t hours   = (int32_t) ((timestamp_ms / (1000 * 60 * 60)));
     U_CX_PORT_PRINTF("[%02" PRId32 ":%02" PRId32 ":%02" PRId32 ".%03" PRId32"]",
                      hours, minutes, seconds, ms);
+#endif
 }
 
 void uCxLogDisable(void)