Fix Java.Interop reference in Mono.Posix.csproj (#60)

Mono.Posix build failed because it recursively requires Java.Interop
reference (due to missing IJavaPeerable in the references).

Adding Java.Interop and System.Runtime fixes the issue (and this is
how Mono.Android builds, instead of adding <ProjectReference> ...).

Author: atsushieno

src/Mono.Posix/Mono.Posix.csproj

@@ -238,6 +238,14 @@
       <Name>Mono.Android</Name>
       <Private>False</Private>
     </ProjectReference>
+    <Reference Include="Java.Interop">
+      <HintPath>$(OutputPath)..\v1.0\Java.Interop.dll</HintPath>
+      <Private>False</Private>
+    </Reference>
+    <Reference Include="System.Runtime">
+      <HintPath>$(OutputPath)..\v1.0\Facades\System.Runtime.dll</HintPath>
+      <Private>False</Private>
+    </Reference>
   </ItemGroup>
   <PropertyGroup>
     <XANativeLibsDir>$(OutputPath)\..\..\..\xbuild\Xamarin\Android\lib</XANativeLibsDir>