staging: android: ion: mark symbols static where possible

We get 4 warnings when building kernel with W=1:
drivers/staging/android/ion/ion_carveout_heap.c:36:17: warning: no previous prototype for 'ion_carveout_allocate' [-Wmissing-prototypes]
drivers/staging/android/ion/ion_carveout_heap.c:50:6: warning: no previous prototype for 'ion_carveout_free' [-Wmissing-prototypes]
drivers/staging/android/ion/ion_of.c:28:5: warning: no previous prototype for 'ion_parse_dt_heap_common' [-Wmissing-prototypes]
drivers/staging/android/ion/ion_of.c:54:5: warning: no previous prototype for 'ion_setup_heap_common' [-Wmissing-prototypes]

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
so this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>