[vold] expose binder headers to dependent modules

This allows libincremental_aidl-cpp to be built via cc_library instead
of aidl_interface.

BUG: 181266844
Test: builds
Change-Id: I4f0bc82629c0df758467aa074274b30f9dc6718d
diff --git a/Android.bp b/Android.bp
index fa8f249..f2a1a37 100644
--- a/Android.bp
+++ b/Android.bp
@@ -96,6 +96,9 @@
     whole_static_libs: [
         "libincremental_aidl-cpp",
     ],
+    export_shared_lib_headers: [
+        "libbinder",
+    ],
 }
 
 cc_library_headers {