Fixed hostapd build without l2_packet (e.g., RADIUS server only).

hostapd/Makefile

@@ -157,6 +157,8 @@ endif
 else
 OBJS += ../src/l2_packet/l2_packet_linux.o
 endif
+else
+OBJS += ../src/l2_packet/l2_packet_none.o
 endif