mpv's zsh completion is generated by running mpv itself. This obviously
doesn't work in cross builds. It used to not fail, because the perl
script would ignore the exec error and just produce a useless zsh
completion file. When they fixed that and started failing, our cross
build started failing as well, so we disabled zsh completion.
This change mostly restores original behaviour: create zsh completion
for non-cross builds.
870e25c515