kernel.h: fix kernel-doc warning

Fix kernel-doc warning in kernel.h from commit 7ef88ad5


("BUILD_BUG_ON: make it handle more cases"):

  Warning(include/linux/kernel.h:605): No description found for parameter 'condition'
  Warning(include/linux/kernel.h:605): Excess function parameter 'cond' description in 'BUILD_BUG_ON'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>