Clean up warnings in libstdc++.

Change-Id: Ie71a2fdf227e7a5fc9a446bb1be110da9b3a4280
diff --git a/libstdc++/Android.mk b/libstdc++/Android.mk
index 41e6c4d..c7ed1f5 100644
--- a/libstdc++/Android.mk
+++ b/libstdc++/Android.mk
@@ -9,6 +9,7 @@
 else
     libstdc++_cflags += -DANDROID_SMP=0
 endif
+libstdc++_cflags += -Wall -Wextra -Werror
 
 include $(CLEAR_VARS)