Fix the warning (for perl < 5.21.x) whihch becomes an error
(for perl >= 5.21.x):

defined(@array) is deprecated at kernel/timeconst.pl line 373.
  (Maybe you should just omit the defined()?)

This error pops up on newer systems. It is not related with the change of
the buildroot, but with the host version of perl.
Signed-off-by: Adam Wujek <adam.wujek@cern.ch>