cmake/rocksdb: make sure dependencies build before rocksdb

cmake/rocksdb: make sure dependencies build before rocksdb

some of rocksdb's dependencies may not have built by the time its
ExternalProject starts, so it can fail with missing headers or
libraries. for example, `uring::uring` may itself be an ExternalProject,
and its include directory won't exist until it starts building:

```
[89/1345] Performing configure step for 'rocksdb_ext'
FAILED: src/rocksdb_ext-prefix/src/rocksdb_ext-stamp/rocksdb_ext-configure build/src/rocksdb_ext-prefix/src/rocksdb_ext-stamp/rocksdb_ext-configure
...
CMake Error in CMakeLists.txt:
  Imported target "uring::uring" includes non-existent path

    "build/src/liburing/src/include"

  in its INTERFACE_INCLUDE_DIRECTORIES.  Possible reasons include:

  * The path was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and references files it does not
  provide.
...
[91/1345] Performing download step (git clone) for 'liburing_ext'
Cloning into 'liburing'...
```

use `add_dependencies(rocksdb_ext)` to make sure all of its dependencies
are available before starting the build

Signed-off-by: Casey Bodley <cbodley@redhat.com>