gpttool: turn on -Werror

- Deal with some -Wunused issues

Change-Id: If9a05ccbda862bbd401df0f5d5a1837abedd9b83
diff --git a/gpttool/Android.mk b/gpttool/Android.mk
index b8f9844..64ad945 100644
--- a/gpttool/Android.mk
+++ b/gpttool/Android.mk
@@ -5,6 +5,7 @@
 
 LOCAL_SRC_FILES := gpttool.c
 LOCAL_STATIC_LIBRARIES := libz
+LOCAL_CFLAGS := -Werror
 
 LOCAL_MODULE := gpttool