blk/CMakeLists.txt: generate interface library if `libblk` is disabled

blk/CMakeLists.txt: generate interface library if `libblk` is disabled

If all the libraries which `libblk` wraps (i.e. Bluestore, AIO, SPDK,
ZBD) are disabled, then the build aborts with a linker failure:

```
 c++ [...] -o bin/ceph-mon [...] -lblk [...]
 mold: fatal: library not found: blk
 collect2: error: ld returned 1 exit status
```

This is because `blk` is used (via `os`) but if the `add_library()`
directive is skipped, `cmake` attempts to look up a library with that
name.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>