rust/Cargo: build as rlib for plugin linkage

Build Rust code as an rlib, in addition to a staticlib so plugins
can link with the Rust code.

rust/Cargo.toml.in

@@ -4,7 +4,7 @@ version = "@PACKAGE_VERSION@"
 edition = "2018"
 
 [lib]
-crate-type = ["staticlib"]
+crate-type = ["staticlib", "rlib"]
 path = "@e_rustdir@/src/lib.rs"
 
 [profile.release]