Partial revert of d4cf8c25

"Generally better to have DLLPUBLIC class with some DLLPRIVATE members."  MSVC
over-eagerly also treats the SdStyleSheetPoolBase template specialization as
dllexport and fails to generate code for the ImplInheritanceHelper4() ctor
because the SfxStyleSheetPool base class has no default ctor.

Change-Id: I656c898ebd3d4ca58785084c5aeed4fa670cb7e0