networkd-ipv4acd.c: Use net/if.h for getting IFF_LOOPBACK definition

This helps in avoiding compiling errors on musl. Definition of
IFF_LOOPBACK is the reason for including linux/if_arp.h, this however
could be obtained from net/if.h glibc header equally and makes it
portable as well.

src/network/networkd-ipv4acd.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
-#include <linux/if_arp.h>
+#include <net/if.h> /* IFF_LOOPBACK */
+#include <net/if_arp.h> /* ARPHRD_ETHER */
 
 #include "sd-dhcp-client.h"
 #include "sd-ipv4acd.h"