remove hand full of 'extern ipacc_rtp_direct' definitions

and instead declare it in gsm_data.h
diff --git a/openbsc/include/openbsc/gsm_data.h b/openbsc/include/openbsc/gsm_data.h
index f4e4d21..f3ec9eb 100644
--- a/openbsc/include/openbsc/gsm_data.h
+++ b/openbsc/include/openbsc/gsm_data.h
@@ -542,6 +542,7 @@
 enum gsm_band gsm_band_parse(const char *mhz);
 
 extern void *tall_bsc_ctx;
+extern int ipacc_rtp_direct;
 
 static inline int is_ipaccess_bts(struct gsm_bts *bts)
 {