btrfs: fix SPDX comment in tree-mod-log.h

The header files should use the /* */ comment style, introduced in
commit f3a84ccd ("btrfs: move the tree mod log code into its own
file").
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tree-mod-log.h

-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 
 #ifndef BTRFS_TREE_MOD_LOG_H
 #define BTRFS_TREE_MOD_LOG_H