gsm0808: add message generator for BSSMAP HANDOVER PERFORMED

We currently have no generator function that can generate BSSMAP
HANDOVER PERFORMED messages. Lets add function for this.

Change-Id: I825106858bd89afc9837811b8fed2e8accc82441
Related: OS#3645
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 3fe9dfc..04a9826 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -181,6 +181,7 @@
 gsm0808_create_handover_detect;
 gsm0808_create_handover_complete;
 gsm0808_create_handover_failure;
+gsm0808_create_handover_performed;
 gsm0808_prepend_dtap_header;
 gsm0808_enc_aoip_trasp_addr;
 gsm0808_dec_aoip_trasp_addr;