varlink-cli: build to OUT_DIR

With Rust 1.28.0:

Cargo will now no longer allow you to publish crates with build scripts
that modify the src directory.
The src directory in a crate should be considered to be immutable.

rust-lang/cargo#5584

varlink-cli/build.rs

@@ -1,5 +1,5 @@
 extern crate varlink;
 
 fn main() {
-    varlink::generator::cargo_build_tosource("src/org.varlink.resolver.varlink", false);
+    varlink::generator::cargo_build("src/org.varlink.resolver.varlink");
 }

varlink-cli/src/org_varlink_resolver.rs

@@ -0,0 +1,5 @@
+#![allow(dead_code)]
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+
+include!(concat!(env!("OUT_DIR"), "/org.varlink.resolver.rs"));