AP_HAL_LINUX: example fix travis warning

missing function declaration
implicit cast
some style fix

libraries/AP_HAL_Linux/examples/GPIOTest/GPIOTest.cpp

@@ -5,6 +5,10 @@
 #include <AP_HAL_Linux/AP_HAL_Linux.h>
 #include <AP_Menu/AP_Menu.h>
 
+void setup();
+void loop();
+int parse_gpio_pin_number(uint8_t argc, const Menu::arg *argv);
+
 #define MENU_FUNC(func) FUNCTOR_BIND(&commands, &MenuCommands::func, int8_t, uint8_t, const Menu::arg *)
 
 const AP_HAL::HAL& hal = AP_HAL::get_HAL();