Fix a gcc warning due to order of initialization in Option

Move idx declaration before minValue and maxValue and silence
this last warning.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/ucioption.cpp

@@ -59,9 +59,9 @@ namespace {
 
     std::string name, defaultValue, currentValue;
     OptionType type;
+    size_t idx;
     int minValue, maxValue;
     ComboValues comboValues;
-    size_t idx;
 
     Option();
     Option(const char* defaultValue, OptionType = STRING);