Include <kernel-features.h> explicitly in Linux clock_settime.c

The rewritten clock_settime code (which now supports 64 bit time on systems
with __WORDSIZE == 32)  for Linux now relies on the
__ASSUME_TIME64_SYSCALLS flag set.

Lets explicitly include the header file where it is defined to avoid
any indirect inclusion (which may pose some unwanted API definitions).

Tested with scripts/build-many-glibcs.py script.