arch/h8300: eliminate ptrace.h warnings

Add a "struct task_struct;" stub to arch/h8300's ptrace.h header to
eliminate gcc warnings (gcc version is 8.1.0).

../arch/h8300/include/asm/ptrace.h:32:34: warning: 'struct task_struct' declared inside parameter list will not be visible outside of this definition or declaration
 extern long h8300_get_reg(struct task_struct *task, int regno);
../arch/h8300/include/asm/ptrace.h:33:33: warning: 'struct task_struct' declared inside parameter list will not be visible outside of this definition or declaration
 extern int h8300_put_reg(struct task_struct *task, int regno,

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: uclinux-h8-devel@lists.sourceforge.jp
Signed-off-by: Yoshinori Sato <ysato@users.sourceforge.jp>