Move current_timespec decl to timespec.h

Move current_timespec decl to timespec.h

This change was motivated by the desire to remove the weird
dependency of lib-src/profile.o on src/systime.h.  profile.c
included systime.h only for current_timespec, and this
inclusion required systime.h to have #ifdef emacs in multiple
places and complicated further changes I have in mind.
The current_timespec decl belongs in timespec.h anyway,
and the main effect of this change is to move it there.
* lib-src/profile.c (INLINE): Remove.
Include timespec.h, not systime.h.
* lib/gettime.c (gettime): Prefer clock_gettime to nanotime,
and don’t worry about it failing on a CLOCK_REALTIME arg.
POSIX requires it to succeed and I don’t know of any
counterexamples where the fallbacks would work.
(current_timespec): Move here from src/systime.h.
Nowadays it seems to be better to not have this function
be inline.
* lib/timespec.h: Include arg-nonnull.h.
(current_timespec): New declaration.
(gettime, settime): Declare args to be nonnull.
* lib/gettime.c, lib/timespec.h: Copy from Gnulib.
* src/systime.h: Simplify by assuming ‘emacs’ is defined,
which it always is now.
(current_timespec): Move to lib/timespec.h.