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.

src/nspawn/nspawn.c

@@ -31,7 +31,6 @@
 #include <stdio.h>
 #include <errno.h>
 #include <sys/prctl.h>
-#include <sys/capability.h>
 #include <getopt.h>
 #include <termios.h>
 #include <sys/signalfd.h>
@@ -90,6 +89,7 @@
 #include "base-filesystem.h"
 #include "barrier.h"
 #include "event-util.h"
+#include "capability.h"
 #include "cap-list.h"
 #include "btrfs-util.h"