techpack: audio-extend: Use EXPORT_SYMBOL_GPL()

Commit 7ac68b2a ("modules: only allow symbol_get of
EXPORT_SYMBOL_GPL modules") made it so symbol_request() can only be used
with GPL symbols, thus we ought to use EXPORT_SYMBOL_GPL() here.

Change-Id: Ia1d1bc79c1fa0a8dd6f432d4b05a1e398268f858