Add empty libcommon-cs

This will gradually soak up code shared by libbsc and libmsc.

Change-Id: If34e2bd38a099d0799238337468d56e0305ab8ae
diff --git a/openbsc/src/Makefile.am b/openbsc/src/Makefile.am
index 272292a..cfad7df 100644
--- a/openbsc/src/Makefile.am
+++ b/openbsc/src/Makefile.am
@@ -27,6 +27,7 @@
 	libmsc \
 	libtrau \
 	libfilter \
+	libcommon-cs \
 	$(NULL)
 
 # Conditional Libraries