In 43c27e8, I mistakenly deleted the patch that removed calls to the
statfs() function, which we do not have. This made building the port
with a clean source tree fail.
This commit changes `libuv` to use our statvfs() function instead.
We've had a half-arsed port of libuv inside the cmake port, but let's
just port it properly.
Note that this pins a specific commit (which is currently the latest
commit in their default branch).
