GCC 15: Fix unterminated-string-initialization (#17244)

Fix build errors on Fedora 42 like:

  module/zcommon/zfs_valstr.c:193:16: error: initializer-string for
  array of 'char' truncates NUL terminator but destination lacks
  'nonstring' attribute (3 chars into 2 available)

The arrays in zpool_vdev_os.c and zfs_valstr.c don't need to be
NULL terminated, but we do so to make GCC happy.

Closes: #17242

Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>

cmd/zpool/os/linux/zpool_vdev_os.c

@@ -87,7 +87,8 @@
 
 typedef struct vdev_disk_db_entry
 {
-	char id[24];
+	/* 24 byte name + 1 byte NULL terminator to make GCC happy */
+	char id[25];
 	int sector_size;
 } vdev_disk_db_entry_t;