ptrace: conditionalize compat_ptrace_request

My recent additions to compat_ptrace_request made it mandatory
for CONFIG_COMPAT arch's to define copy_siginfo_from_user32.
This broke some builds, though they all really should get cleaned
up in that way.

Since all the arch's that actually call compat_ptrace_request have
now been cleaned up to use the generic compat_sys_ptrace, we can
avoid the build problems on the crufty arch's by changing the
conditionals on the definition.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>