gsmtap: Hide implementation of gsmtap_inst

 - Use forward decl. of struct gsmtap_inst in header
 - Remove 'static inline' attributes from gsmtap_inst_fd() declaration,
   move function definition to gsmtap_util.c and mark it as deprecated
 - Add gsmtap_inst_fd2() as replacement for gsmtap_inst_fd()

Related: OS#6213
Change-Id: Ibe1a51205a6df764571b6d074e365825555609a5
diff --git a/src/core/libosmocore.map b/src/core/libosmocore.map
index 6c02cd3..e5f8bd8 100644
--- a/src/core/libosmocore.map
+++ b/src/core/libosmocore.map
@@ -39,6 +39,8 @@
 get_value_string;
 get_value_string_or_null;
 gsmtap_gsm_channel_names;
+gsmtap_inst_fd;
+gsmtap_inst_fd2;
 gsmtap_makemsg;
 gsmtap_makemsg_ex;
 gsmtap_send;