gsm0808: Add create functions for BSS_MAP_MSG_PAGING

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

This commit adds a gsm0808_create_paging() function, that generates an A/AoiP
BSS_MAP_MSG_PAGING message.

Change-Id: I9afecf0109305ca5153bf081bb29cd94071dd2b7
diff --git a/tests/gsm0808/gsm0808_test.ok b/tests/gsm0808/gsm0808_test.ok
index 8e2087d..6170a7a 100644
--- a/tests/gsm0808/gsm0808_test.ok
+++ b/tests/gsm0808/gsm0808_test.ok
@@ -14,6 +14,7 @@
 Testing creating Assignment Failure
 Testing creating Assignment Failure (AoIP)
 Testing creating Clear Request
+Testing creating Paging Request
 Testing creating DTAP
 Testing prepend DTAP
 Done