res_pjsip_session: Restore calls to ast_sip_message_apply_transport()

Commit 44bb0858 ("debugging: Add enough
to choke a mule") accidentally removed calls to
ast_sip_message_apply_transport when it was attempting to just add
debugging code.

The kiss of death was saying that there were no functional changes in
the commit comment.

This makes outbound calls that use the 'flow' transport mechanism fail,
since this call is used to relay headers into the outbound INVITE
requests.

ASTERISK-29124 #close

Change-Id: I0f3e32c2e8ac415e30b1d29966d75a1546f0526a