Prefix custom _complete implementation with "function" keyword

For some reason, when `_complete` is aliased before sourcing the plugin,
zsh was blowing up with a parse error. Adding "function" keyword makes
it parse successfully.

See similar issue: https://github.com/robbyrussell/oh-my-zsh/issues/6723#issuecomment-381220834

Fixes GitHub #442

src/strategies/completion.zsh

@@ -86,7 +86,7 @@ _zsh_autosuggest_capture_completion_async() {
 	# https://stackoverflow.com/a/7057118/154703
 	autoload +X _complete
 	functions[_original_complete]=$functions[_complete]
-	_complete () {
+	function _complete() {
 		unset 'compstate[vared]'
 		_original_complete "$@"
 	}