Add ndk_library for libdl.

Test: make checkbuild
Bug: http://b/30465923
Change-Id: I8acf82f319d3b22948868bc5c4c377f2348af266
diff --git a/libdl/Android.bp b/libdl/Android.bp
index 84561d9..273a887 100644
--- a/libdl/Android.bp
+++ b/libdl/Android.bp
@@ -60,3 +60,9 @@
         never: true,
     },
 }
+
+ndk_library {
+    name: "libdl.ndk",
+    symbol_file: "libdl.map.txt",
+    first_version: "9",
+}