um: Move timer-internal.h to non-shared

This file isn't really shared, it's only used on the kernel side,
not on the user side. Remove the include from the user-side and
move the file to a better place.

While at it, rename it to time-internal.h, it's not really just
timers but all kinds of things related to timekeeping.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>