nspawn: remove spurious include of <sys/capability.h>

nspawn: remove spurious include of <sys/capability.h>

It does not use any functions from libcap directly. The CAP_* constants in use
through this file come from "missing.h" which will import <linux/capability.h>
and complement it with CAP_* constants not defined by the current kernel
headers.

Add an explicit import of our "capability.h" since it does use the function
capability_bounding_set_drop from that header file. Previously, that header was
implicitly imported through through "cap-list.h".

Tested that "systemd-nspawn" builds cleanly and works after this change.