[opendbx] New doxygen 1.8.8 default FileParser causes man page generation failure

[Scott K. <de...@ki...>]

Doxygen has, again, made a backward incompatible API change.  It has a new 
default FileParser which won't parse lib/opendbx/api.  For Debian, I added a 
link do lib/opendbx/api.dox (so the correct FileParser is chosen) and updated 
doc/Doxfile.in.  I didn't actually rename lib/opendbx/api since that would have 
meant messing with the libopendbx makefiles.

FYI.

Scott K