Don't explicitly list the tzcode files.

Change-Id: I3ac19899523ac1f5fd2fdd6c942aefce75a964a4
diff --git a/libc/Android.mk b/libc/Android.mk
index ad5fb1c..b0dd5a5 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -512,13 +512,7 @@
 
 include $(CLEAR_VARS)
 
-LOCAL_SRC_FILES := \
-    tzcode/asctime.c \
-    tzcode/difftime.c \
-    tzcode/localtime.c \
-    tzcode/strftime.c \
-    tzcode/strptime.c \
-
+LOCAL_SRC_FILES := $(call all-c-files-under,tzcode)
 LOCAL_CFLAGS := \
     $(libc_common_cflags) \
     -DSTD_INSPIRED=1 \