[CommCoreModule] Add #include <functional> to NotificationsCryptoModule

Summary:
To resolve build issue after updating Xcode from 15.2 to 15.3.

Specifically, looks like there a number of changes to C++ Standard Library: https://developer.apple.com/documentation/xcode-release-notes/xcode-15_3-release-notes#C++-Standard-Library

The relevant piece might be:

The <experimental/algorithm> and <experimental/functional> headers have been removed, since all the contents have been implemented in namespace std.

We weren't previously including <experimental/functional>, but now it requires us to include <functional> in order for it to recognize std::function.

We include <functional> elsewhere in the codebase, so maybe it was getting resolved by coincidence or something in the past? Either way, this change fixes the iOS build.

Depends on D11261

Test Plan:
Before: didn't build
After: does build

Reviewers: ashoat, ginsu, tomek, marcin, varun

Reviewed By: ginsu, marcin

Differential Revision: https://phab.comm.dev/D11262