lapdm: Make lapdm_datalink_for_sapi public

This API allows you to get the lapdm_datalink for a SAPI. It's needed in
the lapdm_test, so make it public.
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 9d15d66..3c5025d 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -172,6 +172,7 @@
 lapdm_channel_set_l1;
 lapdm_channel_set_l3;
 lapdm_channel_set_mode;
+lapdm_datalink_for_sapi;
 lapdm_entity_exit;
 lapdm_entity_init;
 lapdm_entity_reset;