Makefile: Include hnbgw in SUBDIRS

Change-Id: I04616434cf8b5e67b67803040c3a03e4a0e3d182
diff --git a/Makefile b/Makefile
index 91c826c..1d5e670 100644
--- a/Makefile
+++ b/Makefile
@@ -24,6 +24,7 @@
 	gbproxy \
 	ggsn_tests \
 	hlr \
+	hnbgw \
 	hnodeb \
 	mgw \
 	mme \