libopenmpt: update to 0.5.2.

ChangeLog:

### libopenmpt 0.5.2 (2020-08-30)

*  [**Change**] `Makefile` `CONFIG=emscripten` now supports
    `EMSCRIPTEN_TARGET=all` which provides WebAssembly as well as fallback to
    JavaScript in a single build.

*  [**Regression**] `Makefile` `CONFIG=emscripten` does not support
    `EMSCRIPTEN_TARGET=asmjs` or `EMSCRIPTEN_TARGET=asmjs128m` any more because
    support has been removed from current Emscripten versions.
*  [**Regression**] Support for Emscripten versions older than 1.39.7 has been
    removed.

*  PP20: The first few bytes of some files were not decompressed properly,
    making some files unplayable (depending on the original format).

(fcambus)