Add an ndk_headers module for jni.h.

Change-Id: Ic0a7c246bcc47fc3f0b593f9c27cef53b2f24956
diff --git a/Android.bp b/Android.bp
index 52ae21c..59ddbdd 100644
--- a/Android.bp
+++ b/Android.bp
@@ -69,6 +69,13 @@
     stl: "c++_static",
 }
 
+ndk_headers {
+    name: "ndk_jni.h",
+    from: "include/nativehelper",
+    to: "",
+    srcs: ["include/nativehelper/jni.h"],
+}
+
 //
 // Tests.
 //