n-dhcp4: revert "all: remove unnecessary <netinet/ether.h> includes"

This file has no relevance for NetworkManager. And we should not deviate
from upstream n-dhcp4 which we include via git-subtree. Revert the
change.

This partly reverts commit 7f4a7bf433.

shared/n-dhcp4/src/test-run-client.c

@@ -11,6 +11,7 @@
 #include <errno.h>
 #include <getopt.h>
 #include <net/if.h>
+#include <netinet/ether.h>
 #include <netinet/in.h>
 #include <poll.h>
 #include <stdbool.h>