libosmogb: Port BSSGP flow control from openbsc/laforge/bssgp_fc branch

This code is supposed to implement the BSSGP flow control algorithm,
both for the per-BSS and for the per-MS flow control.

The code currently has no test cases, they will come in a separate
commit.
diff --git a/src/gb/libosmogb.map b/src/gb/libosmogb.map
index a41a91a..8e9f372 100644
--- a/src/gb/libosmogb.map
+++ b/src/gb/libosmogb.map
@@ -2,6 +2,8 @@
 global:
 bssgp_cause_str;
 bssgp_create_cell_id;
+bssgp_fc_in;
+bssgp_fc_ms_init;
 bssgp_msgb_alloc;
 bssgp_msgb_tlli_put;
 bssgp_parse_cell_id;