parisc: includecheck fix: signal.c

fix the following 'make includecheck' warning:

  arch/parisc/kernel/signal.c: linux/compat.h is included more than once.

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>