[PL] SD: Use forward declarations.

ProcessLib/SmallDeformation/CreateSmallDeformationProcess.h

@@ -9,7 +9,24 @@
 
 #pragma once
 
-#include "ProcessLib/Process.h"
+#include <memory>
+#include <vector>
+
+namespace BaseLib
+{
+class ConfigTree;
+}
+namespace MeshLib
+{
+class Mesh;
+}
+namespace ProcessLib
+{
+class AbstractJacobianAssembler;
+struct ParameterBase;
+class Process;
+class ProcessVariable;
+}
 
 namespace ProcessLib
 {

ProcessLib/SmallDeformation/SmallDeformationProcessData.h

@@ -9,6 +9,14 @@
 
 #pragma once
 
+namespace MaterialLib
+{
+namespace Solids
+{
+template <int DisplacementDim>
+struct MechanicsBase;
+}
+}
 namespace ProcessLib
 {
 namespace SmallDeformation