Add libjsoncpp_headers target

Test: build
Bug: 139546461
Change-Id: Id0fcb8c843be04f8d513dae7228cc40de303073d
diff --git a/Android.bp b/Android.bp
index 568cfd3..c574f6f 100644
--- a/Android.bp
+++ b/Android.bp
@@ -49,3 +49,9 @@
     sdk_version: "9",
     stl: "libc++_static",
 }
+
+cc_library_headers {
+    name: "libjsoncpp_headers",
+    export_include_dirs: ["include"],
+    host_supported: true,
+}