encoding: Provide an overload for the gsm_7bit_encode and a simple test

This is required for encoding the SMS header using the alpha numeric
rules.

Reviewed-by: Jacob Erlbeck <jerlbeck@sysmocom.de>
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index b2278f1..8a020db 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -134,6 +134,7 @@
 gsm_7bit_decode;
 gsm_7bit_decode_hdr;
 gsm_7bit_encode;
+gsm_7bit_encode_oct;
 
 gsm_arfcn2band;
 gsm_arfcn2freq10;