`audio/x-wav`: Add missing sub-class relationship with `application/x-riff`

Citations:

• IBM/Microsoft Multimedia Programming Interface and Data Specifications 1.0 http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/Docs/riffmci.pdf#page=36

  "Most of these file formats are based on the Resource Interchange File Format (RIFF). This chapter describes the following file formats: Waveform Audio File Format (WAVE)"

• https://docs.microsoft.com/en-us/windows/win32/xaudio2/resource-interchange-file-format--riff-

  "This overview describes the Resource Interchange File Format (RIFF), which is used in .wav files."

• Library of Congress https://www.loc.gov/preservation/digital/formats/fdd/fdd000001.shtml

  "Subtype of: RIFF, Resource Interchange File Format (RIFF)"

• Any WAVE file :)

  [okeeblow@emi#shared-mime-info] xxd -l 16 tests/mime-detection/test.wav
  00000000: 5249 4646 ac58 0100 5741 5645 666d 7420  RIFF.X..WAVEfmt