Added the black hole directory to the Doxygen list of files.

doc/Doxyfile.in

@@ -776,6 +776,7 @@ INPUT		       += @top_srcdir@/src/entropy_floor/EAGLE
 INPUT		       += @top_srcdir@/src/star_formation/EAGLE
 INPUT		       += @top_srcdir@/src/tracers/EAGLE
 INPUT		       += @top_srcdir@/src/stars/EAGLE
+INPUT		       += @top_srcdir@/src/black_holes/Default
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses