AP_Airspeed: example fix travis warning

missing function declaration
implicit cast
some style fix

libraries/AP_Airspeed/examples/Airspeed/Airspeed.cpp

@@ -23,6 +23,9 @@
 #include <AP_HAL/AP_HAL.h>
 #include <AP_BoardConfig/AP_BoardConfig.h>
 
+void setup();
+void loop();
+
 const AP_HAL::HAL& hal = AP_HAL::get_HAL();
 
 float temperature;
@@ -65,7 +68,7 @@ void loop(void)
         airspeed.get_temperature(temperature);
 
         hal.console->printf("airspeed %5.2f temperature %6.2f healthy = %u\n",
-                            airspeed.get_airspeed(), temperature, airspeed.healthy());
+                            (double)airspeed.get_airspeed(), (double)temperature, airspeed.healthy());
     }
     hal.scheduler->delay(1);
 }