tokio-xmpp: add build script to work around build fail on rustc 1.48

tokio-xmpp/build.rs

@@ -0,0 +1,11 @@
+use rustc_version::version;
+
+fn main() {
+    let version = version().unwrap();
+
+    for major in 1..=version.major {
+        for minor in 0..=version.minor {
+            println!("cargo:rustc-cfg=rustc_least_{}_{}", major, minor);
+        }
+    }
+}