[vim] treat cmx files as JSON

Change-Id: I9df2cd2b89ba5c9594c2d1de73293f16263d0958
diff --git a/vim/fuchsia.vim b/vim/fuchsia.vim
index 30feee5..dcf9132 100644
--- a/vim/fuchsia.vim
+++ b/vim/fuchsia.vim
@@ -91,6 +91,7 @@
     au!
     autocmd BufRead,BufNewFile * call FuchsiaBuffer()
     autocmd FileType cpp call FuchsiaCppBuffer()
+    autocmd BufNewFile,BufRead *.cmx set syntax=json
   augroup END
 
 endif