flacenc: add md5 checksum to STREAMINFO header

Submitted by hzt..@..il.com

Description

I'm using Sound-Juicer on Fedora 27 with GStreamer Core Library version 1.12.4. The created FLAC files are missing the MD5 signature in their header, tested with flac 1.3.2, see below:

flac -t Disc_1_-01-Waiting_for_a_Break.flac
Disc_1-01-_Waiting_for_a_Break.flac: WARNING, cannot check MD5 signature since it was unset in the STREAMINFO
ok

Apparently, this was already reported in the past, but not fixed:

https://bugzilla.redhat.com/show_bug.cgi?id=961881
https://bugzilla.gnome.org/show_bug.cgi?id=727802
https://bugzilla.gnome.org/show_bug.cgi?id=785558

Version: 1.12.4