uml: fix compile for x86-64

Randy Dunlap reports that we get

  arch/x86/um/shared/sysdep/ptrace.h:7:20: error: redefinition of 'regs_return_value'
  arch/x86/um/shared/sysdep/ptrace.h:7:20: note: previous definition of 'regs_return_value' was here

when compiling UML for x86-64.

Stephen Rothwell root-caused it and says:

 "Caused by commit d7e7528b

 ("Audit: push audit success and retcode
  into arch ptrace.h") (another patch that was never in linux-next :-().

  This file now needs protection against double inclusion."

so let's do as the man says.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Analyzed-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>