basic: Add BCACHEFS magic

basic: Add BCACHEFS magic

Import magic.h from Linux 6.9 to get the definition of
BCACHEFS_SUPER_MAGIC. Update filesystems-gperf.gperf to add knowledge of
bcachefs.

This fixes the following error building against a bleeding edge kernel.
```
src/basic/meson.build:234:8: ERROR: Problem encountered: Unknown filesystems defined in kernel headers:

Filesystem found in kernel header but not in filesystems-gperf.gperf: BCACHEFS_SUPER_MAGIC
```