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

Source: matiec
Section: devel
Priority: standard
Maintainer: edouard <edouard.tisserant@gmail.com>
Build-Depends: debhelper (>= 5), bison, flex
Standards-Version: 3.7.2

Package: matiec
Architecture: any
Depends: ${shlibs:Depends}, ${misc:Depends}
Description: The IEC to ANSI-C compiler
 Compiles ST/IL/SFC code into ANSI-C code