[master] Unnecessary header removed from iface_mgr.cc

  - Compilation fix for FreeBSD

src/lib/dhcp/iface_mgr.cc

@@ -18,7 +18,6 @@
 #include <string.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
-#include <net/if.h>
 
 #include <dhcp/dhcp4.h>
 #include <dhcp/dhcp6.h>