gsm0480: code dup: introduce and use gsm0480_l3hdr_push()

Add function gsm0480_l3hdr_push() to push a struct gsm48_hdr to the start of a
msgb. Use in gsm0480.c and gsm0411_utils.c. Further callers of the new function
will follow in openbsc as well as another libosmocore patch for ussd.

Change-Id: I54fce6053ab8362015686fe22dbcd38bf1366700
3 files changed