video: adf: export the adf_attachment_allow symbol to modules.

There are no in-tree users of adf_attachment_allow, but out-of-tree
modules want to use it. It looks like this function should be
EXPORT_SYMBOL.

Change-Id: Iad522dc5d32ac09fec6483bbc317db8ecae12e97
Signed-off-by: Alistair Strachan <alistair.strachan@imgtec.com>