x86_64: fix headers_install

A bug in headers_install for ARCH=x86_64 yields an asm/ directory full of
files all of which are using the same #ifdef guard, "__ASM_STUB_" with no
postfix.  So the second and later asm files #included in the same C file
(often through standard headers like ioctl.h) yields no symbols.

Strangeness with the Ubuntu 'tell me if I support something that's not
explcitly mentioned in POSIX, and I'll strip it out' shell, I believe.

We don't need the 'export' but we do need a semicolon at the end of the
FNAME line:

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Rob Landley <rob@landley.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>