Removed unused 'have_pt_gnu_stack' variable.

Reported by David Binderman <dcb314@hotmail.com>
Signed-off-by: David Rientjes <rientjes@cs.washington.edu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>