rpcrt4: Dynamically load iphlpapi

[Juan Lang]

> Why manually load iphlpapi here instead of using the automatic delayed
> import mechanism like you used for ws2_32? I assume it was because you
> reasoned that a hard dependency was unnecessary because the code has a
> fallback path, but I think you needed to make this clear in the
> changelog.

I can try to make the changelog clearer, but that wasn't my
motivation.  As I mentioned in the email with the patch, iphlpapi is
only needed once per process by rpcrt4, so I manually load it so that
I can also unload it as soon as it's done.  This is in contrast to
ws2_32, where iphlpapi may be needed more than once.
--Juan