[dxbc] Add gs_vertex_in to the entry point interface

Fixes invalid SPIR-V generated in the Blacksmith demo (#267).

src/dxbc/dxbc_compiler.cpp

@@ -5790,6 +5790,8 @@ namespace dxvk {
     m_perVertexIn = m_module.newVar(
       ptrTypeId, spv::StorageClassInput);
     m_module.setDebugName(m_perVertexIn, varName);
+    
+    m_entryPointInterfaces.push_back(m_perVertexIn);
   }