#84 test.performance CString dependencies

[Anonymous]

The following includes are required by the test.performance, but only really needed if FASTFORMAT_INCL_FASTFORMAT_SINK_HPP_CSTRING is defined.
This results in compilation failures using the Express edition of Visual Studio.

# include <mfcstl/mfcstl.hpp>
# include <mfcstl/string/grab_cstring_buffer.hpp>
# include <fastformat/sinks/CString.hpp>

Suggestion here is to #ifdef FASTFORMAT_INCL_FASTFORMAT_SINK_HPP_CSTRING for the inclusions as well.