[ML] Fixed linking issue under macOS when BUILD_SHARED_LIBS=ON.

Linker error was:

Undefined symbols for architecture arm64:
  "MaterialLib::Solids::Ehlers::StateVariables<2>::getEquivalentPlasticStrain() const", referenced from:
      vtable for MaterialLib::Solids::Ehlers::StateVariables<2> in unity_0_cxx.cxx.o
  "MaterialLib::Solids::Ehlers::StateVariables<3>::getEquivalentPlasticStrain() const", referenced from:
      vtable for MaterialLib::Solids::Ehlers::StateVariables<3> in
      unity_0_cxx.cxx.o

MaterialLib/SolidModels/Ehlers.cpp

@@ -842,6 +842,9 @@ SolidEhlers<DisplacementDim>::getInternalVariables() const
 template class SolidEhlers<2>;
 template class SolidEhlers<3>;
 
+template struct StateVariables<2>;
+template struct StateVariables<3>;
+
 template <>
 MathLib::KelvinVector::KelvinMatrixType<3> sOdotS<3>(
     MathLib::KelvinVector::KelvinVectorType<3> const& v)

MaterialLib/SolidModels/Ehlers.h

@@ -24,11 +24,10 @@
 
 #include "BaseLib/Error.h"
 #include "MathLib/KelvinVector.h"
+#include "MechanicsBase.h"
 #include "NumLib/NewtonRaphson.h"
 #include "ParameterLib/Parameter.h"
 
-#include "MechanicsBase.h"
-
 namespace MaterialLib
 {
 namespace Solids
@@ -250,8 +249,8 @@ struct StateVariables
     Damage damage_prev;                      ///< \copydoc damage
 
 #ifndef NDEBUG
-    friend std::ostream& operator<<(
-        std::ostream& os, StateVariables<DisplacementDim> const& m)
+    friend std::ostream& operator<<(std::ostream& os,
+                                    StateVariables<DisplacementDim> const& m)
     {
         os << "State:\n"
            << "eps_p_D: " << m.eps_p.D << "\n"
@@ -358,6 +357,9 @@ private:
 
 extern template class SolidEhlers<2>;
 extern template class SolidEhlers<3>;
+
+extern template struct StateVariables<2>;
+extern template struct StateVariables<3>;
 }  // namespace Ehlers
 }  // namespace Solids
 }  // namespace MaterialLib