Include appropriate header file in drivers/pcsp/pcsp_input.c because it
defines function whose prototype definitions are present in
drivers/pcsp/pcsp_input.h.

This eliminates the following warning in drivers/pcsp/pcsp_input.c:
sound/drivers/pcsp/pcsp_input.c:42:6: warning: no previous prototype for ‘pcspkr_stop_sound’ [-Wmissing-prototypes]
sound/drivers/pcsp/pcsp_input.c:80:5: warning: no previous prototype for ‘pcspkr_input_init’ [-Wmissing-prototypes]
sound/drivers/pcsp/pcsp_input.c:110:5: warning: no previous prototype for ‘pcspkr_input_remove’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>