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/linux/Kbuild b/include/linux/Kbuild
index b6fbb25..71d70d1 100644
--- a/include/linux/Kbuild
+++ b/include/linux/Kbuild
@@ -166,7 +166,7 @@
 unifdef-y += adb.h
 unifdef-y += adfs_fs.h
 unifdef-y += agpgart.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 += apm_bios.h