archs: replace unifdef-y with header-y

unifdef-y and header-y have same semantic, so drop unifdef-y

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/arch/arm/include/asm/Kbuild b/arch/arm/include/asm/Kbuild
index 73237bd..6550db3 100644
--- a/arch/arm/include/asm/Kbuild
+++ b/arch/arm/include/asm/Kbuild
@@ -1,3 +1,3 @@
 include include/asm-generic/Kbuild.asm
 
-unifdef-y += hwcap.h
+header-y += hwcap.h