[Mono.Posix] Sign with mono.pub (#78)

Sign `Mono.Posix.dll` with `mono.pub`, not with `product.snk`.

This is required because the existing Xamarin.Android SDK signs
`Mono.Posix.dll` with `mono.pub`, so the same signing key needs to be
used so that existing assembly references are not broken because a
new/different signing key is used.

Author: jonpryor

src/Mono.Posix/Mono.Posix.csproj

@@ -13,11 +13,11 @@
     <AndroidResgenFile>Resources\Resource.designer.cs</AndroidResgenFile>
     <AssemblyName>Mono.Posix</AssemblyName>
     <SignAssembly>true</SignAssembly>
-    <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
   </PropertyGroup>
   <Import Project="..\..\Configuration.props" />
   <PropertyGroup>
     <TargetFrameworkVersion>$(AndroidFrameworkVersion)</TargetFrameworkVersion>
+    <AssemblyOriginatorKeyFile>$(MonoSourceFullPath)\mcs\class\mono.pub</AssemblyOriginatorKeyFile>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
     <DebugSymbols>true</DebugSymbols>