zsh-completion: complete makechrootpkg arguments after *-build --

[yan12125]

I'm motivated to create this as I often type extra-x86_64-build -- -I xxx.pkg.tar.zst, and unable to completing the package filename is annoying.

Note that I'm far from a talented zsh programmar. This patch works for me and I didn't see other issues for some time, but there might be hidden bugs.

Demonstration:

$ extra-x86_64-build -- -
-D  -- Bind directory into build chroot as read-only
-I  -- Install a package into the working copy
-T  -- Build in a temporary directory
-U  -- Run makepkg as a specified user
-c  -- Clean the chroot before building
-d  -- Bind directory into build chroot as read-write
-h  -- Display usage
-l  -- The directory to use as the working copy
-n  -- Run namcap on the package
-r  -- The chroot dir to use
-u  -- Update the working copy of the chroot before building

[anthraxx]

applied via 66bbddc8c2da82035e6520541f654ddb84039756 thanks