pinctrl: pinctrl-single: add header include guard

Add a header include guard just in case.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Link: https://lore.kernel.org/r/20190720115858.7015-1-yamada.masahiro@socionext.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>