kbuild: fix detection of CONFIG_FRAME_WARN=0

The checking of CONFIG_FRAME_WARN in the top level Makefile forgot to
actually derefence the variable thus leading to an always true check.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Andi Kleen <ak@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>