powerpc/ptrace: Add missing <linux/regset.h> include

ptrace-decl.h uses user_regset_get2_fn (among other things) from
regset.h. While all current users of ptrace-decl.h include regset.h
before it anyway, it adds an implicit ordering dependency and breaks
source tooling that tries to inspect ptrace-decl.h by itself.
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230616034846.311705-3-bgray@linux.ibm.com

arch/powerpc/kernel/ptrace/ptrace-decl.h

 /* SPDX-License-Identifier: GPL-2.0-or-later */
 
+#include <linux/regset.h>
+
 /*
  * Set of msr bits that gdb can change on behalf of a process.
  */