Added IFlatbufferObject.cs file to project for .net.

Before edition, IFlatbufferObject.cs file wasn't in the project and building the project throwed error.

net/FlatBuffers/FlatBuffers.csproj

@@ -37,6 +37,7 @@
     <Compile Include="ByteBuffer.cs" />
     <Compile Include="FlatBufferBuilder.cs" />
     <Compile Include="FlatBufferConstants.cs" />
+    <Compile Include="IFlatbufferObject.cs" />
     <Compile Include="Offset.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Struct.cs" />