asm-generic: support clock_adjtime() in <asm-generic/unistd.h>

A syscall was added without being added to asm-generic, which
makes tile (and presumably score and unicore32) break.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
Cc: Chen Liqin <liqin.chen@sunplusct.com>
Cc: Lennox Wu <lennox.wu@gmail.com>