libopenmpt: update to 0.5.8.

### libopenmpt 0.5.8 (2021-04-11)

*  [**Sec**] Possible null-pointer dereference read caused by a sequence of
    `openmpt::module::read`, `openmpt::module::set_position_order_row` pointing
    to an invalid pattern, and another `openmpt::module::read` call. To trigger
    the crash, pattern 0 must not exist in the file and the tick speed before
    the position jump must be lower than the initial speed of the module.
    (r14530)

*  [**Bug**] `libopenmpt.pc` did not list required system libraries `ole32.lib`
    and `rpcrt4.lib` on Windows in `Libs.Private` field for static builds.
*  [**Bug**] libopenmpt 0.5.7 broke seeking in some subsongs.

*  The built-in LFO plugin did not load the correct initial LFO frequency.
*  IT command S7x (instrument control) is now supported when seeking with
    sample sync enabled.
*  libopenmpt_ext `play_note` was cutting of channels even when there were
    plenty of free channels to use.

*  mpg123: Update to v1.26.5 (2021-03-22).

(fcambus)