Allow *.spx glob for audio/x-speex+ogg

Currently, *.spx is associated with "raw" speex, i.e. without an ogg
container, while it is also often used for audio/x-speex+ogg. The latter
currently only allows the *.oga and *.ogg globs.

See also
https://bugs.freedesktop.org/show_bug.cgi?id=5299
https://bugs.freedesktop.org/show_bug.cgi?id=53376

The libspeex speexenc/speexdec reference implementations use ogg
containers, and use the *.spx extension, see e.g.
https://linux.die.net/man/1/speexdec

"raw" speex and speex+ogg are covered by their content matches, i.e.
'OggS'@0 + 'Speex   '@28 vs. 'Speex   '@0.
Apparently the only application capable of creating "raw" speex is the xiph
sampleenc, see https://github.com/xiph/speex/blob/master/doc/sampleenc.c

Fixes #103
1 job for master in 2 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #2578134
build:meson

00:02:27