lib: descend into sub directories only when it is necessary

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/lib/lzo/Makefile b/lib/lzo/Makefile
index dd853ea..2936544 100644
--- a/lib/lzo/Makefile
+++ b/lib/lzo/Makefile
@@ -5,4 +5,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_LZO) += lzo1x_decompress.o
+obj-y += lzo1x_decompress.o