Now that ratbagd is part of libratbag, the DBus interface is the preferred way
to interact with a device. Shipping libratbag as a .so file means we have to
worry about two ABIs when it's unlikely that anyone ever uses the libratbag.so
directly.
Make it a static library internally instead.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>