xfs: fix fs/xfs/xfs_log.c:1740:39: error: 'B_TRUE' undeclared

Commit 667a9291 "xfs: Remove boolean_t typedef completely." didn't.

Remove a stray B_TRUE that breaks CONFIG_XFS_DEBUG=y.

Signed-off-by: Ben Myers <bpm@sgi.com>
Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>