Fix broken APC submodule

The php-ext/apc submodule was pointing at [a commit hash][1] which doesn't exist any more in the repo.

[1]: <https://github.com/nickl-/pecl-apc/commit/4321b4f0b5535aaa224a6a5c83797f4c2fa6f3b4>

It seems like this commit has been re-written, so this pull request updates it to point to
[the correct SHA][2], which has the same commit message and commit details, but actually exists.

[2]: <https://github.com/nickl-/pecl-apc/commit/40bf2fd37352f0c078bebb609487d75d700a3a34>