build: disable zsh completions by default, fixes e.g. cross compilation

The Perl script generating the completions actually invokes mpv, and it
runs during the build. This is not sane and breaks at least cross
compilation.

As a workaround, disable the completions by default for now.

wscript

@@ -61,6 +61,7 @@ build_options = [
         'name': '--zsh-comp',
         'desc': 'zsh completion',
         'func': check_true,
+        'default': 'disable',
     }, {
         'name': '--macosx-bundle',
         'desc': 'compilation of a Mac OS X Application bundle',