Pack target misses .pri files for Windows targets

[clairernovotny]

When using msbuild /t:pack to generate a NuPkg on a multi-targeted project that includes Windows outputs (uap10.0, win8, wpa81, etc) that also contain a .pri file, the generated NuSpec used to create the package does not include the .pri file. That will lead to errors loading resources.

To repro:
Check out this commit from Zeroconf:
https://github.com/onovotny/Zeroconf/tree/39d47e8fea22a1eb19938392e0bc8f903181cf02

Run the following command msbuild /t:pack /p:Configuration=Release

Look at the NuSpec, it's missing the .pri files.

[clairernovotny]

/cc @emgarten @yishaigalatzer @rrelyea

[srivatsn]

Moved to the NuGet repo.