udev: Factor out SDL_EVDEV_GuessDeviceClass

This works on capability bitfields that can either come from udev or
from ioctls, so it is equally applicable to both udev and non-udev
input device detection.

Signed-off-by: Simon McVittie <smcv@collabora.com>

CMakeLists.txt

@@ -1266,6 +1266,7 @@ elseif(UNIX AND NOT APPLE AND NOT ANDROID AND NOT RISCOS)
     endif()
 
     # Always compiled for Linux, unconditionally:
+    set(SOURCE_FILES ${SOURCE_FILES} "${SDL2_SOURCE_DIR}/src/core/linux/SDL_evdev_capabilities.c")
     set(SOURCE_FILES ${SOURCE_FILES} "${SDL2_SOURCE_DIR}/src/core/linux/SDL_threadprio.c")
 
     # src/core/unix/*.c is included in a generic if(UNIX) section, elsewhere.