Windows: guid_to_string() is only discarded when macro-usbi_dbg used

Windows: guid_to_string() is only discarded when macro-usbi_dbg used

* usbi_dbg encloses all references to guid_to_string
* MinGW/cygwin warn about an unused function, so the #if
  squelches this warning
* MSVC6 uses a variadic function form of usbi_dbg instead
  of a macro, so the compiler still "sees" guid_to_string
  and it therefore needs to always be defined for MSVC6,
  even if it's only a stub.
* So we define it if usbi_dbg is used OR if MSVC6 is used.

Signed-off-by: Michael Plante <michael.plante@gmail.com>