Fix soundtouch recipe include path
When built with autotools or cmake, the libsoundtouch project defines the path '-I${includedir}/soundtouch' in its .pc file. This is the same path that is also used in Ubuntu and Fedora Linux distributions packages. This patch adds this path to the meson generated project using the version 2.3.2-4 of the meson wrap for libsoundtouch.
Edited by Loïc Le Page