[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