kbuild: use in-kernel unifdef

Let headers_install use in-kernel unifdef

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/scripts/Makefile.headersinst b/scripts/Makefile.headersinst
index 12e1daf..8c02d2d 100644
--- a/scripts/Makefile.headersinst
+++ b/scripts/Makefile.headersinst
@@ -7,7 +7,7 @@
 #
 # ==========================================================================
 
-UNIFDEF := unifdef -U__KERNEL__
+UNIFDEF := scripts/unifdef -U__KERNEL__
 
 # Eliminate the contents of (and inclusions of) compiler.h
 HDRSED  := sed 	-e "s/ inline / __inline__ /g" \