background_music: Fix build with MacPorts. Closes: #1528

powf() can't be used with constexpr.

src/background_music/Loudness.h

@@ -43,7 +43,7 @@ struct Loudness
         float weight = 0;
     };
 
-    static constexpr float get_weight(const float seconds)
+    static float get_weight(const float seconds)
     {
         const float relative = aud::clamp(seconds, perception_peak_seconds,
                                           perception_center_seconds) /