include: replace unifdef-y with header-y

unifdef-y and header-y has same semantic.
So there is no need to have both.

Drop the unifdef-y variant and sort all lines again

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

include/linux/nfsd/Kbuild

@@ -1,6 +1,6 @@
-unifdef-y += const.h
-unifdef-y += debug.h
-unifdef-y += export.h
-unifdef-y += nfsfh.h
-unifdef-y += stats.h
-unifdef-y += syscall.h
+header-y += const.h
+header-y += debug.h
+header-y += export.h
+header-y += nfsfh.h
+header-y += stats.h
+header-y += syscall.h