crtools: don't include pstree.h in parasite-syscall.h

pstree and parasite are two different subsystems, so it "should" not
depend on each other.

parasite-syscall uses the pid struct, so this patch moves it in a
separate header.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>