Changes

gsm/protocol/gsm_48_103.h: new header file

3GPP TS 48.103 prescribes a set of fixed RTP payload type numbers
for use on GSM AoIP interface - however, until now there was no
Osmocom header file that formally captures these spec definitions.
Fix this omission.

Change-Id: Id41d4dffd0eb562df4fc948b237d629e858e495b