ARM: 8920/1: share get_signal_page from signal.c to process.c

The get_signal_page() function is defined in signal.c and used in
process.c but there is no shared definition. Add one in signal.h to
silence the following warning:

arch/arm/kernel/signal.c:683:13: warning: symbol 'get_signal_page' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>