hidapi/SDL_hidapijoystick.c: fix build in C90 mode:

src/joystick/hidapi/SDL_hidapijoystick.c: In function 'HIDAPI_InitializeDiscovery':
src/joystick/hidapi/SDL_hidapijoystick.c:281: error: 'true' undeclared (first use in this function)
src/joystick/hidapi/SDL_hidapijoystick.c:281: error: (Each undeclared identifier is reported only once
src/joystick/hidapi/SDL_hidapijoystick.c:281: error: for each function it appears in.)
src/joystick/hidapi/SDL_hidapijoystick.c: In function 'HIDAPI_UpdateDiscovery':
src/joystick/hidapi/SDL_hidapijoystick.c:339: error: 'true' undeclared (first use in this function)
src/joystick/hidapi/SDL_hidapijoystick.c:341: error: ISO C90 forbids mixed declarations and code
1 file changed