Fix missing libhashkit.so in PHP tarball for cflinuxfs5

[ramonskie]

Problem

On Ubuntu 24.04 (cflinuxfs5), the libmemcached package was split: libmemcached.so.11 now dynamically links against libhashkit.so.2 (shipped as the separate libhashkit2t64 package). On Ubuntu 22.04 (cflinuxfs4) libhashkit was statically linked into libmemcached.so.11, so this dependency was invisible.

setupTar() was already copying libmemcached.so* into the PHP tarball but omitted libhashkit.so*, causing memcached.so to fail at runtime on cflinuxfs5:

PHP Startup: Unable to load dynamic library 'memcached.so'
(libhashkit.so.2: cannot open shared object file: No such file or directory)

This manifested as a Composer failure in the php-buildpack integration tests (cflinuxfs5 only):

Your requirements could not be resolved to an installable set of packages.
  Problem 1
    - Root composer.json requires PHP extension ext-memcached * but it is missing from your system.

Fix

Copy libhashkit.so* alongside libmemcached.so* in setupTar() — a single unconditional line, no stack-specific branching needed.

libhashkit.so.2 is present on both stacks (libhashkit-dev is pulled in transitively by libmemcached-dev on Ubuntu 22.04 as well), so the copy is safe for cflinuxfs4 builds too.

Testing

• go test ./internal/recipe/... passes
• PHP tarballs for cflinuxfs5 need to be rebuilt after this lands for the fix to take effect in the buildpack CI