Input: sparc - i8042-sparcio.h: fix unused kbd_res warning

kbd_res is used only when CONFIG_PCI is defined; condition its declaration as
well. This fixes the following compilation warning:

  drivers/input/serio/i8042-sparcio.h:20:25: warning: ‘kbd_res’ defined but not used [-Wunused-variable]

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>