Replace meson join_paths() by '/'

A new '/' operator was introduced in Meson 0.49 to replace join_paths():
 - <https://mesonbuild.com/Reference-manual_functions.html#join_paths>
 - <https://mesonbuild.com/Syntax.html#string-path-building>

Refs #5658 <https://github.com/Genymobile/scrcpy/pull/5658>