ANDROID: ion: fix export symbol type

In commit d872160f ("staging: ion: make system and contig heaps
modular") a new symbol was exported, but it should have been set as a
_GPL symbol.

Fix this up by properly changing the export type.

Bug: 133508579
Cc: Sandeep Patil <sspatil@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I6595d3a3dc12698c16e514d945256e082b942652