Enable dot in doxygen.

Doxyfile

@@ -1541,7 +1541,7 @@ HIDE_UNDOC_RELATIONS   = YES
 # toolkit from AT&T and Lucent Bell Labs. The other options in this section
 # have no effect if this option is set to NO (the default)
 
-HAVE_DOT               = NO
+HAVE_DOT               = YES
 
 # The DOT_NUM_THREADS specifies the number of dot invocations doxygen is
 # allowed to run in parallel. When set to 0 (the default) doxygen will