Use -Werror in external/jsoncpp

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I7549ac243195508ab7d287413cb797ee254614ea
diff --git a/Android.bp b/Android.bp
index 8240f6b..d0550f1 100644
--- a/Android.bp
+++ b/Android.bp
@@ -16,6 +16,9 @@
 
     export_include_dirs: ["include"],
 
-    cflags: ["-DJSON_USE_EXCEPTION=0"],
-
+    cflags: [
+        "-DJSON_USE_EXCEPTION=0",
+        "-Wall",
+        "-Werror",
+    ],
 }