rust.inc: normalize to expanded 4 space tabs

I had previously mixed using hard tabs, 8 spaces, and 4 spaces (due to
editor defaulting to hard tabs and the occasional change to 4 spaces to
work on python code).

Try to avoid that with a vim modeline

recipes/rust/rust.inc

@@ -1,3 +1,4 @@
+# ex: sts=4 et sw=4 ts=8
 inherit rust
 inherit rust-installer
 require rust-shared-source.inc