Add CHANGELOG entry for libbpf#939

Add a CHANGELOG entry for pull request libbpf#939, which adjusted the
PerfBufferBuilder type to work with anything implementing the MapCore
trait instead of strictly requiring a Map object.

Signed-off-by: Daniel Müller <[email protected]>

Author: d-e-s-o

libbpf-rs/CHANGELOG.md

@@ -4,6 +4,7 @@ Unreleased
   - Deprecated `Program::get_id_by_fd`
 - Renamed `Program::get_fd_by_id` to `fd_from_id`
   - Deprecated `Program::get_fd_by_id`
+- Adjusted `PerfBufferBuilder` to work with `MapCore` objects
 
 
 0.24.4