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>