Mark media_plugin_headers as vendor_available

The headers should be available to vendors since vendors implement the
headers.

Bug: 37342627
Test: BOARD_VNDK_VERSION=current m -j camera.device@1.0-impl.vendor
Change-Id: I9936764039f2a5388707bdc093c869ec3551faad
diff --git a/headers/Android.bp b/headers/Android.bp
index 1ea1ea5..bcbbbc3 100644
--- a/headers/Android.bp
+++ b/headers/Android.bp
@@ -1,5 +1,6 @@
 cc_library_headers {
     name: "media_plugin_headers",
+    vendor_available: true,
     export_include_dirs: ["media_plugin"],
     header_libs: [
         "libstagefright_headers",