lib: descend into sub directories only when it is necessary

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/lib/zlib/Makefile b/lib/zlib/Makefile
index 1e9e04d..2fba95f 100644
--- a/lib/zlib/Makefile
+++ b/lib/zlib/Makefile
@@ -5,4 +5,4 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_ZLIB) += zlib.o
+obj-y += zlib.o