ARM: 7892/1: Fix warning for V7M builds

Fixes a harmless warning when building for V7M (!MMU):
 arch/arm/kernel/traps.c:859:123: warning: 'kuser_init' defined but not used [-Wunused-function]

By making the stub static inline instead of just static.

Fixes: f6f91b0d ('ARM: allow kuser helpers to be removed from the vector page')

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>