gsm0808: Add create functions for CIPHER MODE COMMAND

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

This commit adds a gsm0808_create_cipher() function, that generates an A/AoiP
CIPHER MODE COMMAND message.

Change-Id: I8eb1c357860c3e740b0f5d17e1c256bc87920958
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index cf0a7fe..786bf08 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -128,6 +128,7 @@
 gsm0808_create_ass_compl;
 gsm0808_create_assignment_failure;
 gsm0808_create_ass_fail;
+gsm0808_create_cipher;
 gsm0808_create_cipher_complete;
 gsm0808_create_cipher_reject;
 gsm0808_create_classmark_update;