uapi: add missing install of userio.h

While commit 5523662e

 ("Input: add userio module") added userio.h
under the uapi/ directory, it forgot to add the header file to Kbuild.
Thus, the file was missing from header installation.

Signed-off-by: Naohiro Aota <naota@elisp.net>
Reviewed-by: Lyude Paul <thatslyude@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>