ARM: Fix nommu.c build warning

The 0-day kernel build robot found this new warning:

arch/arm/mm/nommu.c:303:17: warning: 'struct proc_info_list' declared inside parameter list [enabled by default]
arch/arm/mm/nommu.c:303:17: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]

Fix it by including the appropriate header.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>