Add missing `add_subdirectory()` call for "cleanup" (#925)

Since `absl::Cleanup` is now public, it should also be included
in the `absl/CMakeLists.txt` file.

Signed-off-by: Christian Blichmann <cblichmann@google.com>

absl/CMakeLists.txt

@@ -16,6 +16,7 @@
 
 
 add_subdirectory(base)
 add_subdirectory(base)
 add_subdirectory(algorithm)
 add_subdirectory(algorithm)
+add_subdirectory(cleanup)
 add_subdirectory(container)
 add_subdirectory(container)
 add_subdirectory(debugging)
 add_subdirectory(debugging)
 add_subdirectory(flags)
 add_subdirectory(flags)