Include wavpack/wavpack.h for WavpackContext

[daschuer]

Depending on the Wavpack version, WavpackContext is defined as "void" or "struct WavpackContext".
Clang build aborts with an error if that does not match which has happened here:
#2414

[daschuer]

Still failing, but differently:

In file included from /Users/travis/build/mixxxdj/mixxx/src/sources/soundsourceproxy.cpp:31:
In file included from /Users/travis/build/mixxxdj/mixxx/src/sources/soundsourcewv.h:6:
/usr/local/include/wavpack/wavpack.h:46:3: error: typedef redefinition with different types ('struct ChunkHeader' vs 'struct ChunkHeader')
} ChunkHeader;
  ^
/Applications/Xcode-11.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/AIFF.h:84:41: note: previous definition is here
typedef struct ChunkHeader              ChunkHeader;
                                        ^
In file included from /Users/travis/build/mixxxdj/mixxx/src/sources/soundsourceproxy.cpp:4:

[uklotzde]

• Use static_cast<WavpackContext*>(m_wpc) in .cpp file
• Build failures Warnings for Qt 5.14 on macOS are unrelated (need to be fixed separately)

[uklotzde]

LGTM. Thanks for fixing this dependency issue!