fix vim syntastic: --parse-only => -Z parse-only

when saving .rs files under vim
do not fail to run the syntax checker
error: Unrecognized option: 'parse-only'.

due to this commit
953f294ea30253bb5578e3c895d17fcc97c20dce
which removed the deprecated flag --parse-only

src/etc/vim/syntax_checkers/rust/rustc.vim

@@ -14,7 +14,7 @@ let s:save_cpo = &cpo
 set cpo&vim
 
 function! SyntaxCheckers_rust_rustc_GetLocList() dict
-    let makeprg = self.makeprgBuild({ 'args': '--parse-only' })
+    let makeprg = self.makeprgBuild({ 'args': '-Zparse-only' })
 
     let errorformat  =
         \ '%E%f:%l:%c: %\d%#:%\d%# %.%\{-}error:%.%\{-} %m,'   .