Remove warnings from ijar/classfile.cc

Also add -Werror.

Change-Id: I2f9fda288d31bd34735856ade33916c93c45922a
diff --git a/tools/ijar/Android.mk b/tools/ijar/Android.mk
index 36f000c..8b2a02c 100644
--- a/tools/ijar/Android.mk
+++ b/tools/ijar/Android.mk
@@ -8,7 +8,7 @@
 include $(CLEAR_VARS)
 LOCAL_CPP_EXTENSION := .cc
 LOCAL_SRC_FILES := classfile.cc ijar.cc zip.cc
-LOCAL_CFLAGS += -Wall
+LOCAL_CFLAGS += -Wall -Werror
 LOCAL_SHARED_LIBRARIES := libz-host
 LOCAL_MODULE := ijar
 # libc++ is not supported for TARGET_BUILD_APPS builds