move 'extern struct gsm_network *bsc_gsmnet" to header file

It's not a good idea to keep extern declarations copied over
half a dozen C files.  Let's move it to a header.

Change-Id: I6f643f1393ba0955d9c0cf1cf78d5c604e7b9451
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 5ba4362..1cf79a5 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -1350,6 +1350,8 @@
 	return 0;
 }
 
+extern struct gsm_network *bsc_gsmnet;
+
 enum bts_gprs_mode bts_gprs_mode_parse(const char *arg, int *valid);
 const char *bts_gprs_mode_name(enum bts_gprs_mode mode);
 int bts_gprs_mode_is_compat(struct gsm_bts *bts, enum bts_gprs_mode mode);