Add media/openmax as an include directory

This will obviate changing Khronos's header files.

Note that all header files in media/openmax have OMX_ prefix, so the
chance of collision is already low.

Test: Compiles
Bug: 63821829
Change-Id: I40cc92409de5091a1e2ed7c89c0012000b20f68f
diff --git a/headers/Android.bp b/headers/Android.bp
index bcbbbc3..82bc8a1 100644
--- a/headers/Android.bp
+++ b/headers/Android.bp
@@ -1,7 +1,10 @@
 cc_library_headers {
     name: "media_plugin_headers",
     vendor_available: true,
-    export_include_dirs: ["media_plugin"],
+    export_include_dirs: [
+        "media_plugin",
+        "media_plugin/media/openmax",
+    ],
     header_libs: [
         "libstagefright_headers",
         "libcutils_headers",