ethernet_interface: Fix configuration

Include config.h to satisfy #ifdefs used in code.
Fixes config 'persist-mac' and 'uboot-env' having no effect.

Change-Id: I27a4984dc069871dc757388494fcf145069a6143
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
diff --git a/src/ethernet_interface.cpp b/src/ethernet_interface.cpp
index 2851a1b..0427363 100644
--- a/src/ethernet_interface.cpp
+++ b/src/ethernet_interface.cpp
@@ -1,3 +1,4 @@
+#include "config.h"
 
 #include "ethernet_interface.hpp"