fix C compilation warning about function prototype in __SET_EXTERNAL

Example warning:
./POUS.c:1220:3: error: function declaration isn't a prototype [-Werror=strict-prototypes]
__SET_EXTERNAL(data__->,KN_AUT,,__GET_VAR(data__->KEY_AUT.STATE_OUT));
^
compilation terminated due to -Wfatal-errors.
cc1: all warnings being treated as errors

matiec (2009.0-%DATEVERSION%) unstable; urgency=low

  * Initial release (Closes: #nnnn)  <nnnn is the bug number of your ITP>

 -- edouard <edouard.tisserant@gmail.com>  Wed, 21 Nov 2007 17:01:50 +0100