Newer glibc releases have removed SIGUNUSED from bits/signum.h which
breaks building criu on those systems:

https://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=8082d91e1c449e0cb137468b731004a5e605c8c6

SIGSYS returns the same value as SIGUNUSED and testing has shown
that criu still successfully completes zdtm with this patch.
Signed-off-by: Adrian Reber <areber@redhat.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>