Make LINE_NUMBER_HERE workaround conditional on LL_WINDOWS.

The referencing code is all within LL_WINDOWS conditionals, so the dummy
variable must be enclosed that way too -- otherwise we get
unreferenced-variable warnings-as-errors on other platforms.

indra/llui/llxuiparser.cpp

@@ -58,7 +58,9 @@ static 	LLInitParam::Parser::parser_inspect_func_map_t sSimpleXUIInspectFuncs;
 
 const char* NO_VALUE_MARKER = "no_value";
 
+#ifdef LL_WINDOWS
 const S32 LINE_NUMBER_HERE = 0;
+#endif
 
 struct MaxOccursValues : public LLInitParam::TypeValuesHelper<U32, MaxOccursValues>
 {