arch/sh/kernel/kgdb.c: add missing #include <linux/sched.h>

arch/sh/kernel/kgdb.c: add missing #include <linux/sched.h>

  arch/sh/kernel/kgdb.c: In function 'sleeping_thread_to_gdb_regs':
  arch/sh/kernel/kgdb.c:225:32: error: implicit declaration of function 'task_stack_page' [-Werror=implicit-function-declaration]
  arch/sh/kernel/kgdb.c:242:23: error: dereferencing pointer to incomplete type
  arch/sh/kernel/kgdb.c:243:22: error: dereferencing pointer to incomplete type
  arch/sh/kernel/kgdb.c: In function 'singlestep_trap_handler':
  arch/sh/kernel/kgdb.c:310:27: error: 'SIGTRAP' undeclared (first use in this function)
  arch/sh/kernel/kgdb.c:310:27: note: each undeclared identifier is reported only once for each function it appears in

This was introduced by commit 16559ae48c76 ("kgdb: remove #include
<linux/serial_8250.h> from kgdb.h").

[[email protected]: reworded and reformatted]
Signed-off-by: Wanlong Gao <[email protected]>
Signed-off-by: Geert Uytterhoeven <[email protected]>
Reported-by: Fengguang Wu <[email protected]>
Acked-by: Greg Kroah-Hartman <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>