Fix uninitialized warning and remove header pollution (#803)

* Fix uninitialized warning and remove header pollution

* Nits

* IWYU

* More IWYU

include/audio/audio_interpolation.hpp

@@ -16,9 +16,9 @@ namespace Audio::Interpolation {
 	using StereoFrame16 = Audio::DSPMixer::StereoFrame<s16>;
 
 	struct State {
-		// Two historical samples.
-		std::array<s16, 2> xn1 = {};  //< x[n-1]
-		std::array<s16, 2> xn2 = {};  //< x[n-2]
+		// Two history samples.
+		std::array<s16, 2> xn1 = {};  // x[n-1]
+		std::array<s16, 2> xn2 = {};  // x[n-2]
 		// Current fractional position.
 		u64 fposition = 0;
 	};