gsm_7bit_encode_n(): fix integer overflow in gsm_septets2octets()

Using 'uint8_t' for the length argument is definitely a bad idea.
Because of this, packing more than 255 septets would not work as
expected.  Deprecate the old function and use 'size_t' instead.

Change-Id: Ib1aac538afeb0a5c76a1df472d555139a496e12e
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index c314c20..584d761 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -478,6 +478,7 @@
 
 gsm_milenage;
 gsm_septet_encode;
+gsm_septet_pack;
 gsm_septets2octets;
 
 lapd_dl_exit;