Add empty libcommon-cs

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

Change-Id: If34e2bd38a099d0799238337468d56e0305ab8ae
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index b18ecc1..093d42f 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -214,6 +214,7 @@
     src/libcommon/Makefile
     src/libfilter/Makefile
     src/libiu/Makefile
+    src/libcommon-cs/Makefile
     src/osmo-nitb/Makefile
     src/osmo-bsc/Makefile
     src/osmo-bsc_nat/Makefile