powerpc/ptrace: Move declaration of ptrace_get_reg() and ptrace_set_reg()

ptrace_get_reg() and ptrace_set_reg() are only used internally by
ptrace.

Move them in arch/powerpc/kernel/ptrace/ptrace-decl.h

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/376c258267aeae54a4423bc4a2e107a9611f0039.1597770847.git.christophe.leroy@csgroup.eu