rpi: compile in the common udev code

In a recent commit 37d38d932c, "rpi: Use
common udev_input for input device handling", the rpi-backend was made
to use the common udev code.

It just forgot to actually build the common udev code into the
rpi-backend.

Cc: Jonas Ådahl <jadahl@gmail.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/Makefile.am

@@ -218,6 +218,8 @@ rpi_backend_la_SOURCES =			\
 	rpi-renderer.c				\
 	rpi-renderer.h				\
 	rpi-bcm-stubs.h 			\
+	udev-seat.c				\
+	udev-seat.h				\
 	evdev.c					\
 	evdev.h					\
 	evdev-touchpad.c