Fix building the mono version

diff --git a/Source/Core/BuilderMono.csproj b/Source/Core/BuilderMono.csproj
index 980fed4b..2419a7b0 100644
--- a/Source/Core/BuilderMono.csproj
+++ b/Source/Core/BuilderMono.csproj
@@ -304,6 +304,8 @@
     <Compile Include="Rendering\Vector4.cs" />
     <Compile Include="Rendering\VertexBuffer.cs" />
     <Compile Include="Rendering\VisualSlopeHandle.cs" />
+    <Compile Include="SRB2\LuaMobjStructure.cs" />
+    <Compile Include="SRB2\LuaParser.cs" />
     <Compile Include="VisualModes\VisualSlope.cs" />
     <Compile Include="Windows\ILinedefEditForm.cs" />
     <Compile Include="Windows\ISectorEditForm.cs" />