ARM: versatile: Add missing ENDPROC to headsmp.S

Once the ENDPROC is in place, BSYM() in not longer necessary
to get correct pointer to versatile_secondary_startup().

Tested-by: Jon Medhurst <tixy@linaro.org>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Acked-by: Dave Martin <dave.martin@linaro.org>