gstlal/doc/Doxyfile.in: generate a tagfile for doxygen cross links

gstlal/doc/Doxyfile.in

@@ -1640,7 +1640,7 @@ TAGFILES               =
 # When a file name is specified after GENERATE_TAGFILE, doxygen will create
 # a tag file that is based on the input files it reads.
 
-GENERATE_TAGFILE       =
+GENERATE_TAGFILE       = gstlaldoxy.tag
 
 # If the ALLEXTERNALS tag is set to YES all external classes will be listed
 # in the class index. If set to NO only the inherited external classes