fix(windows): make GetIpNetEntry2 optional

[Arqu]

We had CI fail recently which I traced to a bump to netdev from 0.45 to 0.46.
Ref https://github.com/n0-computer/iroh/actions/runs/32749963007/job/97507327668?pr=4487

Seems like it's a well known issue in the Wine circles and GetIpNetEntry2 is stubbed out making the process fail.
Apparently there's work to get it supported in the bleeding edge of Wine/Proton, but this should make it safe for older setup to work too.

I kept the change as minimal as possible.

Here's a quick matrix after testing on our CI instance after this patch

Built from this branch for x86_64-pc-windows-gnu, run under Wine 11.0 on Linux
(Ubuntu 22.04, fresh WINEPREFIX). Baseline = main (v0.46.1), unmodified.

#	Binary	Baseline	This PR
1	examples/default_gateway	❌ exit 1	✅ exit 0
2	examples/list_interfaces	❌ exit 1	✅ exit 0
3	cargo test --lib (20 tests)	—	✅ 20 passed, 0 failed

Before:

wine: Call from ... to unimplemented function iphlpapi.dll.GetIpNetEntry2, aborting
Unhandled exception: unimplemented function iphlpapi.dll.GetIpNetEntry2 called in 64-bit code

After: default_gateway completes with:

Default Gateway
        MAC Address: 00:00:00:00:00:00
        IPv4: [<redacted>]
        IPv6: [fe80::1]

[shellrow]

Thanks for the fix and the detailed testing!
This looks great.