types: use siginfo_t from signal.h for rt_signalfn_t declaration

It should fit the sa_sigaction declaration, otherwise compiler
complains about uncasted assignments.

 | restorer.c: In function тАШ__export_restore_taskтАЩ:
 | restorer.c:318:20: error: assignment from incompatible pointer type [-Werror]

So just use siginfo_t here from the system signal.h header.

Signed-off-by: Andrew Grigorev <andrew@ei-grad.ru>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>