gsm0808: Add create functions for BSS_MAP_MSG_ASSIGMENT_RQST

gsm0808.h/c lacks functionality to generate BSS_MAP_MSG_ASSIGMENT_RQST messages.
These messages are required if the code is used in an MSC implementation.

This commit adds a gsm0808_create_assignment() function, that generates an
A/AoiP BSS_MAP_MSG_PAGING message.

Change-Id: I4d1d455a1e1cf95407e23ded7b7defbcf2dd6ff0
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index ec23418..a4e6083 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -124,6 +124,7 @@
 gsm0808_att_tlvdef;
 gsm0808_bssap_name;
 gsm0808_bssmap_name;
+gsm0808_create_ass;
 gsm0808_create_assignment_completed;
 gsm0808_create_ass_compl;
 gsm0808_create_assignment_failure;