gpio: move pthread header inclusion to libsoc_gpio.h

Fixes following compile time error:

libsoc_gpio.h:21:2: error: unknown type name 'pthread_t'

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Merge pull request #8 from yegorich/pthread

gpio: move pthread header inclusion to libsoc_gpio.h