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 <[e-mail]>
Signed-off-by: Richard Kuo <[e-mail]>

arch/hexagon/kernel/kgdb.c

···
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  */
 
+#include <linux/irq.h>
+#include <linux/sched.h>
 #include <linux/kdebug.h>
 #include <linux/kgdb.h>