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
```
anon_inodefs, {ANON_INODE_FS_MAGIC}
autofs, {AUTOFS_SUPER_MAGIC}
balloon-kvm, {BALLOON_KVM_MAGIC}
+bcachefs, {BCACHEFS_SUPER_MAGIC}
bdev, {BDEVFS_MAGIC}
binder, {BINDERFS_SUPER_MAGIC}
binfmt_misc, {BINFMTFS_MAGIC}
#else
assert_cc(NTFS3_SUPER_MAGIC == 0x7366746e);
#endif
+
+/* Added in Linux commit e2f48c48090dea172c0c571101041de64634dae5. Remove when next sync'd */
+#ifndef BCACHEFS_SUPER_MAGIC
+# define BCACHEFS_SUPER_MAGIC 0xca451a4e
+#else
+assert_cc(BCACHEFS_SUPER_MAGIC == 0xca451a4e)
+#endif