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

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

They do not use any functions from libcap directly. The CAP_* constants in use
through these files come from "missing.h" which will import <linux/capability.h>
and complement it with CAP_* constants not defined by the current kernel
headers. The "missing.h" header is imported through "util.h" which gets
imported in "logind.h".

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