With the changes in 3b3ea346,
"Remove all uses of deprecated AVOptions API", av_opt_flag_is_set
was broken, since it now uses av_opt_find, which doesn't return
named constants unless a unit to look for the constant in is given.

This broke enabling LATM encapsulated AAC output in RTP.
Signed-off-by: Martin Storsjö <martin@martin.st>