aconfig: fix broken auto-generated Rust code

Fix syntax error in auto-generated Rust code that prevents the code from
compiling.

For reference, the error was:

---- 8< ----
error: expected `where`, `{`, `(`, or `;` after struct name, found keyword `impl`
 --> .../src/lib.rs:6:1
  |
6 | impl FlagProvider {
  | ^^^^ expected `where`, `{`, `(`, or `;` after struct name
---- >8 ----

Bug: 292052163
Test: manual: create sample based on auto-generated Rust lib
Change-Id: I4c029c0ee16ea3849e7bbe381717a10402c9cb81