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/ipaccess/ipaccess-config.c b/openbsc/src/ipaccess/ipaccess-config.c
index 9d0e2db..368f788 100644
--- a/openbsc/src/ipaccess/ipaccess-config.c
+++ b/openbsc/src/ipaccess/ipaccess-config.c
@@ -896,8 +896,8 @@
 	if (!gsmnet)
 		exit(1);
 
-	bts = gsm_bts_alloc(gsmnet, GSM_BTS_TYPE_NANOBTS, HARDCODED_TSC,
-				HARDCODED_BSIC);
+	bts = gsm_bts_alloc_register(gsmnet, GSM_BTS_TYPE_NANOBTS, HARDCODED_TSC,
+				     HARDCODED_BSIC);
 	/* ip.access supports up to 4 chained TRX */
 	gsm_bts_trx_alloc(bts);
 	gsm_bts_trx_alloc(bts);