libbsc: Move SIs update/generation for a BTS into a separate function.

The code to do that doesn't belong to the control interface, so
abstract it out to a separate function gsm_bts_set_system_infos().

[hfreyther: Fix the coding style...]
diff --git a/openbsc/include/openbsc/gsm_data.h b/openbsc/include/openbsc/gsm_data.h
index 30a397c..6f7c8dd 100644
--- a/openbsc/include/openbsc/gsm_data.h
+++ b/openbsc/include/openbsc/gsm_data.h
@@ -438,6 +438,7 @@
 int gsm_bts_has_feature(struct gsm_bts *bts, enum gsm_bts_features feat);
 struct gsm_bts_trx *gsm_bts_trx_by_nr(struct gsm_bts *bts, int nr);
 int gsm_bts_trx_set_system_infos(struct gsm_bts_trx *trx);
+int gsm_bts_set_system_infos(struct gsm_bts *bts);
 
 /* generic E1 line operations for all ISDN-based BTS. */
 extern struct e1inp_line_ops bts_isdn_e1inp_line_ops;