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
FUNCTION_BLOCK HYSTERESIS (* Boolean hysteresis on difference *) (* of REAL inputs, XIN1 - XIN2 *) VAR_INPUT XIN1, XIN2, EPS : REAL; END_VAR VAR_OUTPUT Q : BOOL := 0; END_VAR IF Q THEN IF XIN1 < (XIN2 - EPS) THEN Q := 0; END_IF ; ELSIF XIN1 > (XIN2 + EPS) THEN Q := 1 ; END_IF ;END_FUNCTION_BLOCK