linux(4): Rename linux_timer.h to linux_time.h

To avoid confusing people, rename linux_timer.h to linux_time.h,
as linux_timer.c is the implementation of timer syscalls only,
while linux_time.c contains implementation of all stuff declared
in linux_time.h.

MFC after: 2 weeks

(cherry picked from commit c8a79231a5a74fc4263ec3d18c6519a974efb529)