oggenc: Flush after writing headers

oggenc: Flush after writing headers

Before, header information for ogg format files was sent with the
first encoded packet.

This patch makes it so that it is possible for API users to
differentiate between headers and encoded audio. This is useful, for
example, when creating an audio stream where you want to send one set
of headers for every client that connects and then the encoded stream
of audio.

Signed-off-by: Martin Storsjö <martin@martin.st>