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

They do not use any functions from libcap directly. The CAP_KILL constant in
use by these files comes from <linux/capability.h> imported through
"missing.h".

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

src/machine/machine-dbus.c

@@ -21,7 +21,6 @@
 
 #include <errno.h>
 #include <string.h>
-#include <sys/capability.h>
 #include <arpa/inet.h>
 
 #include "bus-util.h"

src/machine/machined-dbus.c

@@ -23,7 +23,6 @@
 #include <string.h>
 #include <unistd.h>
 #include <pwd.h>
-#include <sys/capability.h>
 
 #include "sd-id128.h"
 #include "sd-messages.h"