staging: android: ion: statify __ion_add_cma_heaps

Fix the following sparse warning:
symbol '__ion_add_cma_heaps' was not declared. Should it be static?

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/ion/ion_cma_heap.c

@@ -106,7 +106,7 @@ static struct ion_heap *__ion_cma_heap_create(struct cma *cma)
 	return &cma_heap->heap;
 }
 
-int __ion_add_cma_heaps(struct cma *cma, void *data)
+static int __ion_add_cma_heaps(struct cma *cma, void *data)
 {
 	struct ion_heap *heap;