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

syntax: glob

.project
*~
*.a
*.orig
.dep
*.Po
autom4te.cache
Makefile
Makefile.in
config.log
stamp-h1
config.status
configure
aclocal.m4



syntax: regexp

\.o$
^iec2c$
^iec2iec$
\.depend$
^stage1_2/iec_bison.cc
^stage1_2/iec_bison.h
^stage1_2/iec_flex.cc
^config/config.h
^test/