asm/neon.h doesn't have a header inclusion guard, but it should
have one for consistency with other headers.

This patch adds a suitable guard.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>