add rootfs mount dir variable to pkg-config

add rootfs mount dir variable to pkg-config

In the case we use an image for rootfs, if we need to do extra mount
from the host to the rootfs, we have to specify the place where the
image is mounted. This value is configured by the user with the
lxc.rootfs.mount otherwise defaulting to @LXCROOTFSMOUNT@. Let's
export this variable to pkg-config, so the user can use it to build
a correct path to the rootfs.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>