xtensa: pull signal definitions from signal-defs.h

This fixes the following build error in the current linux-next:

include/linux/signal.h:261:2: error: unknown type name '__sigrestore_t'
make[2]: *** [arch/xtensa/kernel/asm-offsets.s] Error 1
make[1]: *** [prepare0] Error 2
make: *** [sub-make] Error 2

that appeared after 32dae82 'consolidate kernel-side struct sigaction declarations'

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>