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/TODO-RELEASE b/TODO-RELEASE
index 2f9903d..b67161d 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -7,4 +7,5 @@
 # If any interfaces have been added since the last public release: c:r:a + 1.
 # If any interfaces have been removed or changed since the last public release: c:r:0.
 #library	what			description / commit summary line
-core      ADD       osmo_sock_multiaddr_{add,del}_local_addr()
\ No newline at end of file
+core      ADD       osmo_sock_multiaddr_{add,del}_local_addr()
+core      ADD       gsmtap_inst_fd2() core, DEPRECATE gsmtap_inst_fd()