Doxyfile: predefine OPENSLIDE_SIMPLIFY_HEADERS
Doxyfile: predefine OPENSLIDE_SIMPLIFY_HEADERS

Newer versions of Doxygen started trying to document the unimplemented
prefetch hint functions.  OPENSLIDE_SIMPLIFY_HEADERS suppresses these
from openslide.h, so just define that.

openslide/openslide,openslide/openslide,openslide/openslide,openslide/openslide