frame_relay: Export osmo_fr_network_free()

The function already existed, it was just not declared and not
exported.

Change-Id: I3f2a7e2fd1a4eda6bbed8510cf82951d73bb3f84
diff --git a/src/gb/libosmogb.map b/src/gb/libosmogb.map
index fc8ab45..ed9b822 100644
--- a/src/gb/libosmogb.map
+++ b/src/gb/libosmogb.map
@@ -100,6 +100,7 @@
 bssgp_bvc_fsm_get_max_pdu_len;
 
 osmo_fr_network_alloc;
+osmo_fr_network_free;
 osmo_fr_link_alloc;
 osmo_fr_link_free;
 osmo_fr_dlc_alloc;