Exclude System.Security.Cryptography for UWP projects

[mieliespoor]

The System.Security.Cryptography namespace should be excluded from UWP assemblies since this is now part of the framework itself.

The following compiler error is observed when trying to build projects trying to calculate a SHA1 hash:

The type 'SHA1Managed' exists in both 'Cimbalino.Toolkit.Core, Version=2.5.2.0, Culture=neutral, PublicKeyToken=null' and 'System.Security.Cryptography.Algorithms, Version=4.3.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'