ANDROID: SoC: core: export snd_soc_info_multi_ext as gpl-only

The commit with change-id I26343af2e4a35952d93dc2d5fa0caad2a3e50ffe only
exported a new symbol, it needs to be marked as gpl-only until it lands
upstream to be properly handled.

Cc: Banajit Goswami <bgoswami@codeaurora.org>
Cc: Sudheer Papothi <spapothi@codeaurora.org>
Cc: Meng Wang <mwang@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I07bbe701dfcd3fe84818bde45ccac1cf699fe769