ARM: 9302/1: traps: hide unused functions on NOMMU

ARM: 9302/1: traps: hide unused functions on NOMMU

A couple of functions in this file are only used on MMU-enabled
builds, and never even declared otherwise, causing these build
warnings:

arch/arm/kernel/traps.c:759:6: error: no previous prototype for '__pte_error' [-Werror=missing-prototypes]
arch/arm/kernel/traps.c:764:6: error: no previous prototype for '__pmd_error' [-Werror=missing-prototypes]
arch/arm/kernel/traps.c:769:6: error: no previous prototype for '__pgd_error' [-Werror=missing-prototypes]

Protect these in an #ifdef to avoid the warnings and save a little
bit of .text space.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>