ipaccess: Remove the ipaccess_gsmnet and assume there is a bsc_gsmnet

Assume that there is a bsc_gsmnet and modify the ipaccess-config to
provide this symbol. If a bsc_gsmmnet is not available when linked
the linker will complain and fail.

E.g. give an error like this:
../../src/libbsc/libbsc.a(bts_ipaccess_nanobts.o): In function `ipaccess_sign_link_up':
src/libbsc/bts_ipaccess_nanobts.c:550: undefined reference to `bsc_gsmnet'
diff --git a/openbsc/src/ipaccess/ipaccess-config.c b/openbsc/src/ipaccess/ipaccess-config.c
index 05fdc14..14749d8 100644
--- a/openbsc/src/ipaccess/ipaccess-config.c
+++ b/openbsc/src/ipaccess/ipaccess-config.c
@@ -53,7 +53,7 @@
 #include <osmocom/core/talloc.h>
 #include <osmocom/abis/abis.h>
 
-static struct gsm_network *gsmnet;
+struct gsm_network *bsc_gsmnet;
 
 static int net_listen_testnr;
 static int restart;
@@ -980,11 +980,11 @@
 	}
 	libosmo_abis_init(tall_ctx_config);
 
-	gsmnet = gsm_network_init(1, 1, NULL);
-	if (!gsmnet)
+	bsc_gsmnet = gsm_network_init(1, 1, NULL);
+	if (!bsc_gsmnet)
 		exit(1);
 
-	bts = gsm_bts_alloc_register(gsmnet, GSM_BTS_TYPE_NANOBTS, HARDCODED_TSC,
+	bts = gsm_bts_alloc_register(bsc_gsmnet, GSM_BTS_TYPE_NANOBTS, HARDCODED_TSC,
 				     HARDCODED_BSIC);
 	/* ip.access supports up to 4 chained TRX */
 	gsm_bts_trx_alloc(bts);