gsm0480: Factor out msgb allocation helper function

Change-Id: If25b467481023eadaaf3f78157eceff4b81d24d2
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 3b403c2..312c990 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -97,6 +97,7 @@
 gsm0480_wrap_invoke;
 gsm0480_comp_type_names;
 gsm0480_op_code_names;
+gsm0480_msgb_alloc_name;
 
 gsm0502_calc_paging_group;