ftplugin/debcontrol: Add "setlocal expandtab"

Hello!

I have been running into issue where the Tab character is sometimes inadvertently inserted, especially with Neovim's default which, when I press o to add a new line, intelligently adds whitespace to align with the line above, but erroneously uses a Tab instead.

I added setlocal expandtab to /usr/share/nvim/runtime/syntax/debcontrol.vim as a local workaround.

Thanks!