audio hal: add exports to audio-base.h

Fixes build errors with double definitions of
some constants here and in audio.h

Change-Id: I1569b86216c6502a20ff6f39a991cd6087be8a23
diff --git a/audio/common/2.0/types.hal b/audio/common/2.0/types.hal
index fa06cc4..dd75706 100644
--- a/audio/common/2.0/types.hal
+++ b/audio/common/2.0/types.hal
@@ -55,12 +55,12 @@
 /*
  * Each port has a unique ID or handle allocated by policy manager.
  */
+@export(name="")
 enum AudioHandleConsts {
     AUDIO_IO_HANDLE_NONE = 0,
     AUDIO_MODULE_HANDLE_NONE = 0,
     AUDIO_PORT_HANDLE_NONE = 0,
     AUDIO_PATCH_HANDLE_NONE = 0,
-    AUDIO_HW_SYNC_INVALID = 0
 };
 
 /*