Fixed missing definition of lfs_cache_drop in readonly mode
Fixed missing definition of lfs_cache_drop in readonly mode

Interestingly this was introduced by two different PRs which were not tested
together until pre-release testing:

- Fix lfs_file_seek doesn't update cache properties correctly
- Fix compiler warnings when LFS_READONLY defined

littlefs-project/littlefs,littlefs-project/littlefs