split gsm_data.c in gsm_data_shared.c and gsm_data.c

This facilitates the use of gsm_data.c from osmo-bts
diff --git a/openbsc/src/libcommon/Makefile.am b/openbsc/src/libcommon/Makefile.am
index ed74b44..a832b75 100644
--- a/openbsc/src/libcommon/Makefile.am
+++ b/openbsc/src/libcommon/Makefile.am
@@ -4,4 +4,4 @@
 
 noinst_LIBRARIES = libcommon.a
 
-libcommon_a_SOURCES = bsc_version.c common_vty.c debug.c gsm_data.c socket.c talloc_ctx.c
+libcommon_a_SOURCES = bsc_version.c common_vty.c debug.c gsm_data.c gsm_data_shared.c socket.c talloc_ctx.c