DSP: Implement audio filters (simple, biquad)

src/audio_core/CMakeLists.txt

@@ -2,13 +2,16 @@ set(SRCS
             audio_core.cpp
             codec.cpp
             hle/dsp.cpp
+            hle/filter.cpp
             hle/pipe.cpp
             )
 
 set(HEADERS
             audio_core.h
             codec.h
+            hle/common.h
             hle/dsp.h
+            hle/filter.h
             hle/pipe.h
             sink.h
             )