disable timestamps in the footer of docs by doxygen

The default being 'yes', but this spoils the effort of reproducible
builds for no real gain. (https://wiki.debian.org/ReproducibleBuilds)

doc/Doxyfile.in

@@ -1128,7 +1128,7 @@ HTML_COLORSTYLE_GAMMA  = 80
 # The default value is: YES.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_TIMESTAMP         = YES
+HTML_TIMESTAMP         = NO
 
 # If the HTML_DYNAMIC_SECTIONS tag is set to YES then the generated HTML
 # documentation will contain sections that can be hidden and shown after the