x86: ptrace.c only needs export.h and not the full module.h

Commit cb57a2b4

 ("x86-32: Export
kernel_stack_pointer() for modules") added an include of the
module.h header in conjunction with adding an EXPORT_SYMBOL_GPL
of kernel_stack_pointer.

But module.h should be avoided for simple exports, since it in turn
includes the world.  Swap the module.h for export.h instead.

Cc: Jiri Kosina <trivial@kernel.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Link: http://lkml.kernel.org/r/1360872842-28417-1-git-send-email-paul.gortmaker@windriver.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>