mfd: Remove unneeded version.h include from ab5500

As 'make versioncheck' points out, there's no need for
drivers/mfd/ab5500-core.c to #include <linux/version.h>, so this patch
removes the unneeded include.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>