Make NuGet package copy native binaries on windows only

The props file added to the NuGet package in #821 makes sure the Windows
native binaries are copied to the correct location. However, as pointed out
in #835, this now happens even when the package is installed on a non-Windows
platform.

This can be prevented by adding a condition to the imported ItemGroup that
makes it be ignored when running on a non-Windows platform.

Closes #835

nuget.package/build/LibGit2Sharp.props

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
-	<ItemGroup>
+	<ItemGroup Condition=" '$(OS)' == 'Windows_NT' ">
 		<None Include="$(MSBuildThisFileDirectory)\..\..\lib\net40\NativeBinaries\amd64\git2-e0902fb.dll">
 			<Link>NativeBinaries\amd64\git2-e0902fb.dll</Link>
 			<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>