gpiod.h: include missing <time.h> include

gpiod.h: include missing <time.h> include

gpiod.h uses "struct timespec", but forgets to include the <time.h>
header which defines "struct timespec". This causes a build error with
the musl C library:

In file included from core.c:11:0:
../../include/gpiod.h:232:49: warning: ‘struct timespec’ declared inside parameter list
 typedef int (*gpiod_event_cb)(int, const struct timespec *, void *);

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>