This fixes following error on `make distcheck`
```
Making all in SoundTouch
CXX AAFilter.lo
CXX FIRFilter.lo
CXX FIFOSampleBuffer.lo
CXX RateTransposer.lo
../../../../source/SoundTouch/RateTransposer.cpp:39:10: fatal error: 'InterpolateLinear.h' file not found
^~~~~~~~~~~~~~~~~~~~~
1 error generated.
```