Put variable definition before use.

Though it's technically possible to do it the other way around because of the constexpr, it doesn't make much 'aestetic' sense.

done as part of CURA-12017

include/GCodePathConfig.h

@@ -18,6 +18,8 @@ namespace cura
  */
 struct GCodePathConfig
 {
+    static constexpr double FAN_SPEED_DEFAULT = -1.0;
+
     coord_t z_offset{}; //<! vertical offset from 'full' layer height
     PrintFeatureType type{}; //!< name of the feature type
     coord_t line_width{}; //!< width of the line extruded
@@ -27,7 +29,6 @@ struct GCodePathConfig
     bool is_bridge_path{ false }; //!< whether current config is used when bridging
     double fan_speed{ FAN_SPEED_DEFAULT }; //!< fan speed override for this path, value should be within range 0-100 (inclusive) and ignored otherwise
     double extrusion_mm3_per_mm{ calculateExtrusion() }; //!< current mm^3 filament moved per mm line traversed
-    static constexpr double FAN_SPEED_DEFAULT = -1;
 
     [[nodiscard]] constexpr bool operator==(const GCodePathConfig& other) const noexcept = default;
     [[nodiscard]] constexpr auto operator<=>(const GCodePathConfig& other) const = default;