kbuild: descend into scripts/gcc-plugins/ via scripts/Makefile

Now that 'archprepare' depends on 'scripts', Kbuild can descend into
scripts/gcc-plugins in a more standard way.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
diff --git a/scripts/Makefile b/scripts/Makefile
index b48259d..feb1f71 100644
--- a/scripts/Makefile
+++ b/scripts/Makefile
@@ -36,9 +36,10 @@
 build_unifdef: $(obj)/unifdef
 	@:
 
+subdir-$(CONFIG_GCC_PLUGINS) += gcc-plugins
 subdir-$(CONFIG_MODVERSIONS) += genksyms
 subdir-$(CONFIG_SECURITY_SELINUX) += selinux
 subdir-$(CONFIG_GDB_SCRIPTS) += gdb
 
 # Let clean descend into subdirs
-subdir-	+= basic dtc kconfig mod package gcc-plugins
+subdir-	+= basic dtc kconfig mod package