nwfilter: add a missing define, so libvirtd builds on macos x

The nwfilter code uses ETH_ALEN, which isn't defined on MacOS X.
This is a simple workaround, to add it when missing.

src/nwfilter/nwfilter_learnipaddr.c

@@ -38,6 +38,11 @@
 #include <net/if_arp.h>
 #include <intprops.h>
 
+// Add Linux define missing from OSX
+#ifndef ETH_ALEN
+# define ETH_ALEN       6               /* Octets in one ethernet addr   */
+#endif
+
 #include "internal.h"
 
 #include "buf.h"