input: touchscreen: Fix error due to assumption of CONFIG_FB always being set

synaptics_dsx's synaptics_dsx_rmi_dev.c calls send_sig_info(), but if
CONFIG_FB isn't set, then the implicit linux/sched/signal.h include gets
lost. Explicitly include linux/sched/signal.h to fix this.

Change-Id: I411e39b6bcbe843f64a10447a867ffb64b4222cf