import oap_client into libosmogsm

This imports the code from osmo-msc 6afef893e17bce67e4d4119acd34d480ed03ba77
with minimal changes to make it compile.  Symbol renaming to osmo_
prefix is done separately in a follow-up patch to have a as-clean-as-possible
import first.

Change-Id: I9bc38102318da02d1fe46ef516df3cfd6bf8e3da
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index b44cfd2..6eb60cc 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -493,5 +493,10 @@
 osmo_mncc_names;
 _osmo_mncc_log;
 
+oap_client_encoded;
+oap_client_handle;
+oap_client_init;
+oap_client_register;
+
 local: *;
 };