mk: Always touch libuv.a

libuv.a always looks out of date to the makefile, causing make to
always descend into the libuv makefile, even when there's nothing
to build.

mk/rt.mk

@@ -215,6 +215,7 @@ $$(LIBUV_DIR_$(1))/Release/libuv.a: $$(LIBUV_DEPS) $$(LIBUV_MAKEFILE_$(1)) \
 		BUILDTYPE=Release \
 		NO_LOAD="$$(LIBUV_NO_LOAD)" \
 		V=$$(VERBOSE)
+	$$(Q)touch $$@
 
 endif