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>

include/uapi/linux/Kbuild

@@ -437,6 +437,7 @@ header-y += unistd.h
 header-y += unix_diag.h
 header-y += usbdevice_fs.h
 header-y += usbip.h
+header-y += userio.h
 header-y += utime.h
 header-y += utsname.h
 header-y += uuid.h