kbuild: fix a.out.h export to userspace with O= build.

We need to check for existence of the a.out.h header in the source tree,
not the object tree, if we want it to get the right answer with O=.

Signed-off-by: David Woodhouse <david.woodhouse@intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/include/asm-generic/Kbuild.asm b/include/asm-generic/Kbuild.asm
index 92a6d91..7cd25b8 100644
--- a/include/asm-generic/Kbuild.asm
+++ b/include/asm-generic/Kbuild.asm
@@ -1,6 +1,6 @@
 header-y  += kvm.h
 
-ifeq ($(wildcard include/asm-$(SRCARCH)/a.out.h),include/asm-$(SRCARCH)/a.out.h)
+ifneq ($(wildcard $(srctree)/include/asm-$(SRCARCH)/a.out.h),)
 unifdef-y += a.out.h
 endif
 unifdef-y += auxvec.h