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
```

src/basic/missing_magic.h

@@ -100,3 +100,10 @@ assert_cc(NTFS_SB_MAGIC == 0x5346544e);
 #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