Doxygen: Fix warnings about undocumented compounds

Doxygen: Fix warnings about undocumented compounds

Commit fa3f91e7c4 ("descriptor: Miscellaneous improvements to the
overall source") introduced some union types, however since descriptor.c
is parsed by Doxygen it will complain with the following warnings:

  libusb/descriptor.c:49: warning: Compound string_desc_buf is not documented.
  libusb/descriptor.c:55: warning: Compound bos_desc_buf is not documented.
  libusb/descriptor.c:43: warning: Compound config_desc_buf is not documented.

Fix this by moving the union definitions into libusbi.h.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>