Packaging: mention MIT in the license expression

This project seems to include portions of code under MIT, so formally
any derived work has to follow the terms of both Apache 2 and MIT
licenses.

Author: ForNeVeR

src/FixedMath.NET/FixedMath.NET.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
+    <PackageLicenseExpression>Apache-2.0 AND MIT</PackageLicenseExpression>
   </PropertyGroup>
 
   <ItemGroup>