RCPP-85 fixed gcc clang build ubuntu 24.04

Issue RCPP-85 Fixed missing header in realm.hpp mentioned in RCPP-85. Fixed the following build error in gcc: > cpprealm/link.hpp:190:21: error: expected unqualified-id before ‘)’ token > 190 | managed<T*>() = default; > | ^ > > managed() = default; By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.