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/include/osmocom/gsm/gsm0808.h b/include/osmocom/gsm/gsm0808.h
index 990fd74..a9f610a 100644
--- a/include/osmocom/gsm/gsm0808.h
+++ b/include/osmocom/gsm/gsm0808.h
@@ -42,6 +42,11 @@
 struct msgb *gsm0808_create_classmark_update(const uint8_t *cm2, uint8_t cm2_len,
 					     const uint8_t *cm3, uint8_t cm3_len);
 struct msgb *gsm0808_create_sapi_reject(uint8_t link_id);
+struct msgb *gsm0808_create_ass(const struct gsm0808_channel_type *ct,
+				const uint16_t *cic,
+				const struct sockaddr_storage *ss,
+				const struct gsm0808_speech_codec_list *scl,
+				const uint32_t *ci);
 struct msgb *gsm0808_create_ass_compl(uint8_t rr_cause, uint8_t chosen_channel,
 				      uint8_t encr_alg_id, uint8_t speech_mode,
 				      const struct sockaddr_storage *ss,