Export rpc_unstable header

It'll be used by microdroid payload.

Test: build
Change-Id: I6eaccb55f291a7426ee047bbf6f1f2d211b5b94d
diff --git a/libs/binder/Android.bp b/libs/binder/Android.bp
index 572d948..fccc9fa 100644
--- a/libs/binder/Android.bp
+++ b/libs/binder/Android.bp
@@ -329,6 +329,7 @@
         "libbinder_ndk",
         "libutils",
     ],
+    export_include_dirs: ["include_rpc_unstable"],
 
     // enumerate stable entry points, for apex use
     stubs: {
@@ -342,6 +343,11 @@
     ],
 }
 
+filegroup {
+    name: "libbinder_rpc_unstable_header",
+    srcs: ["include_rpc_unstable/binder_rpc_unstable.hpp"],
+}
+
 // libbinder historically contained additional interfaces that provided specific
 // functionality in the platform but have nothing to do with binder itself. These
 // are moved out of libbinder in order to avoid the overhead of their vtables.