enhancement to support gcc visibility.

These changes are meant to support gcc visibility on mingw platforms - which is basically gcc with windows attributes.

src/basic_iarchive.cpp

@@ -561,7 +561,6 @@ basic_iarchive::load_pointer(
     const basic_pointer_iserializer * (*finder)(
         const boost::serialization::extended_type_info & type_
     )
-
 ){
     return pimpl->load_pointer(*this, t, bpis_ptr, finder);
 }