Plugin: add export to buffer_size function.

 This patch fixes a problem where `mk_plugin_load_symbol` returns 0
 for `_mkp_network_io_buffer_size` function.
Merge pull request #166 from Denisss025/plugin_buff_size_export

Plugin: add export to buffer_size function.
Merge branch 'master' of https://github.com/monkey/monkey into ldpreload