doxygen: Explicitly enable generation of HTML doc
doxygen: Explicitly enable generation of HTML doc

Running 'make docs' with doxygen 1.8.7 gives:
"Warning: No output formats selected! Set at least one of the main
GENERATE_* options to YES."
and even though make is successful, no documentation is generated.
Adding GENERATE_HTML = yes to Doxyfile avoids this.

metabrainz/libmusicbrainz,metabrainz/libmusicbrainz,metabrainz/libmusicbrainz