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