Kernel commit
cb12fd8e0dabb9a1c8aef55a6a41e2c255fcdf4b added pidfs.
Update filesystems-gperf.gperf and missing_magic.h accordingly.
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: PID_FS_MAGIC
```
openpromfs, {OPENPROM_SUPER_MAGIC}
orangefs, {ORANGEFS_DEVREQ_MAGIC}
overlay, {OVERLAYFS_SUPER_MAGIC}
+pidfs, {PID_FS_MAGIC}
pipefs, {PIPEFS_MAGIC}
ppc-cmm, {PPC_CMM_MAGIC}
proc, {PROC_SUPER_MAGIC}
#define DEVMEM_MAGIC 0x454d444d
#endif
+/* cb12fd8e0dabb9a1c8aef55a6a41e2c255fcdf4b (6.8) */
+#ifndef PID_FS_MAGIC
+#define PID_FS_MAGIC 0x50494446
+#endif
+
/* Not in mainline but included in Ubuntu */
#ifndef SHIFTFS_MAGIC
#define SHIFTFS_MAGIC 0x6a656a62