Enable Doxygen to generate a tag file (#410)

doxygen/Doxyfile.in

@@ -2166,7 +2166,7 @@ TAGFILES               =
 # tag file that is based on the input files it reads. See section "Linking to
 # external documentation" for more information about the usage of tag files.
 
-GENERATE_TAGFILE       =
+GENERATE_TAGFILE       = k4a.tagfile
 
 # If the ALLEXTERNALS tag is set to YES, all external class will be listed in
 # the class index. If set to NO, only the inherited external classes will be