Use -Werror in system/vold/tests

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: Ied1f6160124d10d81151d401ed4a0b089816abb3
diff --git a/tests/Android.mk b/tests/Android.mk
index 416e621..b1e309b 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -12,4 +12,5 @@
 LOCAL_MODULE := vold_tests
 LOCAL_MODULE_TAGS := eng tests
 
+LOCAL_CFLAGS := -Wall -Werror
 include $(BUILD_NATIVE_TEST)