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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 Things that we must still check for in stage 3:

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1) Handling of CONSTANTs:

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 1.a) "Any program organization unit attempts to modify the value of a variable that has been declared with the CONSTANT qualifier;"

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 1.b) From table 16.a "The CONSTANT qualifier shall not be used in the declaration of function block instances as described in 2.5.2.1."

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