Makefile: fixed standalone minerd building

Makefile

@@ -52,7 +52,7 @@ darkfid: contracts
 		RUST_TARGET="$(RUST_TARGET)" \
 		RUSTFLAGS="$(RUSTFLAGS)"
 
-minerd:
+minerd: contracts
 	$(MAKE) -C bin/$@ \
 		PREFIX="$(PREFIX)" \
 		CARGO="$(CARGO)" \