hexagon: kernel: kgdb: include related header for pass compiling.

Need include related headers for pass compiling, the related error
(with allmodconfig for v4):

    CC      arch/hexagon/kernel/kgdb.o
  arch/hexagon/kernel/kgdb.c:30: error: invalid use of undefined type 'struct pt_regs'
  arch/hexagon/kernel/kgdb.c:31: error: invalid use of undefined type 'struct pt_regs'
  ...
  arch/hexagon/kernel/kgdb.c:220: error: implicit declaration of function 'local_irq_save'
  arch/hexagon/kernel/kgdb.c:222: error: implicit declaration of function 'local_irq_restore'
  ...

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>