doc: Update Doxyfile

It gives:

Warning: Tag `SYMBOL_CACHE_SIZE' at line 346 of file doc/Doxyfile has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"

So run 'doxygen -u' as suggested.

Signed-off-by: Ran Benita <ran234@gmail.com>