RCPP-85 fixed gcc clang build ubuntu 24.04

[BestITUserEUW]

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;

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[BestITUserEUW]

Tested: with:

gcc version 12.3.0, 13.2.0
clang version: 18.1.3
system: ubuntu 24.04 x86_64