Add a build-rust alias to just rebuild the rust code.

src/rust/tor_rust/include.am

@@ -15,4 +15,10 @@ distclean-rust:
 		$(CARGO) clean --quiet $(CARGO_ONLINE) )
 	rm -rf "$(abs_top_builddir)/src/rust/registry"
 
+if USE_RUST
+build-rust: src/rust/target/release/@TOR_RUST_STATIC_NAME@
+else
+build-rust:
+endif
+
 FORCE: