Microsoft.Win32.Primitives.csproj: Set AllowUnsafeBlocks to true

When building this project on Unix, the Interop.strerror.Unix.cs file
is included which contains p/invoke calls. We need to enable the
AllowUnsafeBlocks property to allow this.
Merge pull request #291 from akoeplinger/patch-1

Microsoft.Win32.Primitives.csproj: Set AllowUnsafeBlocks to true