pstree: Define RLIM_NLIMITS

On PI machine we've got

 |   CC	   protobuf.o
 | pstree.c: In function ‘core_entry_alloc’:
 | pstree.c:36:10: error: ‘RLIM_NLIMITS’ undeclared (first use in this function)

due to old kernel headers. Note I've dropped off
BUG_ON here to localize all things in pstree code,
no need to sprinkle constants.

Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>