time: Allow later version licensing.

The FSF's Licensing and Compliance Lab noted a discrepancy in the
licensing of several files in the glibc package.

When timespect_get.c was impelemented the license did not include
the standard ", or (at your option) any later version." text.

Change the license in timespec_get.c and all copied files to match
the expected license.

This change was previously approved in principle by the FSF in
RT ticket #1316403. And a similar instance was fixed in
commit 46703efa.

(cherry picked from commit 91695ee4)