Drop <sys/user.h> to fix PAGE_SIZE declaration

On some systems PAGE_SIZE is declared as sysconf(_SC_PAGESIZE) in <sys/user.h>
this is non-constant expression, so it cannot be used in type declarations.

This breaks compilation with a very non-obvious error message:

  CC       parasite-syscall.o
In file included from parasite-syscall.c:30:0:
./include/parasite.h:90:8: error: variably modified ‘fds’ at file scope

crtools doesn't uses anything from <sys/user.h>, so we can drop its usage.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>