l2_packet_freebsd: Fix macOS build

ETHER_VLAN_ENCAP_LEN does not seem to be defined in macOS
net/ethernet.h, so define that, if needed, to avoid build issues.

Fixes: 5b21f4861c ("l2_packet_freebsd: Enable receiving priority tagged (VID=0) frames")
Signed-off-by: Jouni Malinen <j@w1.fi>

src/l2_packet/l2_packet_freebsd.c

@@ -30,6 +30,9 @@
 #include "eloop.h"
 #include "l2_packet.h"
 
+#ifndef ETHER_VLAN_ENCAP_LEN
+#define ETHER_VLAN_ENCAP_LEN 4
+#endif
 
 static const u8 pae_group_addr[ETH_ALEN] =
 { 0x01, 0x80, 0xc2, 0x00, 0x00, 0x03 };