json-c: Add libjson to the vendor image

libjson is not part of stock AOSP and needs to be installed into the
vendor image.

Change-Id: I7c170cb7cdf08a41f3494f0c956c9bdc93e89440
diff --git a/Android.mk b/Android.mk
index 575be91..139a10c 100644
--- a/Android.mk
+++ b/Android.mk
@@ -31,4 +31,5 @@
 		random_seed.c
 LOCAL_SHARED_LIBRARIES := libcutils libutils
 LOCAL_MODULE_TAG := optional
+LOCAL_VENDOR_MODULE := true
 include $(BUILD_SHARED_LIBRARY)