gsm_04_08.h: Introduce API osmo_gsm48_rfpowercap2powerclass()

Related: OS#4244
Change-Id: I32e9cc1c2397b44f0d48db2acdf782a821365b63
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 724fe5b..a0e3b32 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -646,6 +646,7 @@
 osmo_gsm48_classmark_a5_name_buf;
 osmo_gsm48_classmark_a5_name_c;
 osmo_gsm48_classmark_update;
+osmo_gsm48_rfpowercap2powerclass;
 
 cbsp_msg_type_names;
 cbsp_iei_names;