doc: Do not add timestamps to Doxygen output

The API documentation is not shipped in any binary packages but it might
in the future. Given that it already contains the project version down
to a git commit if necessary and the Doxygen version, there's really no
need for the timestamp.

doc/Doxyfile.in

@@ -1152,7 +1152,7 @@ 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