lwip: pull in `lwip_ethernet` when `netdev_eth` is present

With out this fix the application or device drivers need to add this
module. Otherwise, compilation of the lwIP adaptation layer will fail
for Ethernet devices.

Makefile.dep

@@ -466,6 +466,9 @@ ifneq (,$(filter lwip,$(USEMODULE)))
   ifeq (,$(filter lwip_tcp lwip_udp lwip_udplite,$(USEMODULE)))
     USEMODULE += lwip_raw
   endif
+  ifneq (,$(filter netdev_eth,$(USEMODULE)))
+    USEMODULE += lwip_ethernet
+  endif
 endif
 
 ifneq (,$(filter lwip_ppp,$(USEMODULE)))