Use -Werror in external/libmojo

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I50e61863d37610e9f6f7420c82c2e861f16eeb02
diff --git a/Android.mk b/Android.mk
index f2f18cb..482194c 100644
--- a/Android.mk
+++ b/Android.mk
@@ -159,6 +159,8 @@
 	mojo/public/cpp/system/watcher.cc \
 
 LOCAL_CFLAGS := \
+	-Wall \
+	-Werror \
 	-Wno-unused-parameter \
 	-Wno-missing-field-initializers \
 	-DMOJO_EDK_LEGACY_PROTOCOL \