[doc] Update doxygen.conf.

This patch is to suppress the following warning on Xubuntu 13.10.

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$ doxygen -u doxygen.conf
Warning: Tag `SYMBOL_CACHE_SIZE' at line 336 of file doxygen.conf has become obsolete.
To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
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According to the message, I did the following command.

    $ doxygen -u doxygen.conf