isac: Work around upstream having no shared library support for MSVC

None of the symbols in webrtc-audio-coding-1 are marked with __declspec(dllexport), rendering the library usable only if it was built with GCC/Clang.

The only fix available (as the pulseaudio copy has not been updated with Google's upstream) is to ensure the fallback builds statically. Although this change will also affect webrtcdsp's dependency on webrtc-audio-processing-1, it does not break its compilation.