ARM: 8916/1: mm: make set_section_perms() static

The set_section_perms() is not defined outside of the
init.c file, so make it static to avoid the following
warning:

arch/arm/mm/init.c:596:6: warning: symbol 'set_section_perms' 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>