rust-common: Ensure sstate signatures have correct dependencues for do_rust_gen_targets

rust-common: Ensure sstate signatures have correct dependencues for do_rust_gen_targets

The do_rust_gen_targets task was not rerunning when the configuration variables it
uses were changed. Add the missing variable dependencies to fix this.

[RP: Split to separate patch and add commit message]
Signed-off-by: Sundeep KOKKONDA <sundeep.kokkonda@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 65ce2d129154d4540cb8ec960971bcf6aa5a9480)
Signed-off-by: Steve Sakoman <steve@sakoman.com>