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

     1

STD_TEST.st

     2

STD_RESSOURCE.c

     3

LOCATED_VARIABLES.h

     4

POUS.c

     5

test

     6

STD_CONF.c

     7

SFC_TEST.st

     8

test.exe

     9

plc.st

    10

POUS.h

    11

VARIABLES.csv