gpio: select ANON_INODES

The build servers found that gpiolib is using ANON_INODES but
has forgotten to select it. Fix this.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 521a2ad6

 ("gpio: add userspace ABI for GPIO line information")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>