games/openbor: fix inline warnings

In file included from source/scriptlib/Interpreter.c:9:
In file included from ./openborscript.h:12:
In file included from source/scriptlib/Interpreter.h:12:
In file included from source/scriptlib/StackedSymbolTable.h:12:
In file included from source/scriptlib/SymbolTable.h:11:
source/scriptlib/ScriptVariant.h:66:23: warning: inline function 'ScriptVariant_Xor' is not defined

[-Wundefined-inline]

inline ScriptVariant *ScriptVariant_Xor( ScriptVariant *svar, ScriptVariant *rightChild );

^

source/scriptlib/Interpreter.c:1090:22: note: used here

BINARYOP(ScriptVariant_Xor);
         ^