avcodec: add constants for KLV pseudo-profile.

avcodec: add constants for KLV pseudo-profile.

There are two different ways KLV is used in MISB specs - sync and async.
The corresponding text (in ST1401) says:

ISO/IEC 13818-1 Table-34 defines a stream_type = 0x15 for “Metadata carried in PES packets,”
and Table 2-22 defines a stream_id = 0xFC for “metadata stream.”

and

In ISO/IEC 13818-1, Table-34 defines a stream_type = 0x06 for “PES packets containing private
data,” and Table 2-22 defines a stream_id = 0xBD for “private_stream_1.”

These constants allow us to distinguish the two cases, as codec profiles.

Signed-off-by: Marton Balint <cus@passwd.hu>