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

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

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

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

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