pinctrl: samsung: Add include guard to local header

The pinctrl-exynos.h header is included only once so till now it did not
require an include guard.  However adding such is harmless and makes
code prepared for more inclusions.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>