make: clean at test/rpc when performing "make clean" from main dir

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Makefile

@@ -222,6 +222,7 @@ clean: clean-built
 	$(Q) $(RM) protobuf-desc-gen.h
 	$(Q) $(MAKE) -C test/zdtm cleandep clean cleanout
 	$(Q) $(MAKE) -C test/libcriu clean
+	$(Q) $(MAKE) -C test/rpc clean
 
 distclean: clean
 	$(E) "  DISTCLEAN"