staging: ion: vm_insert_pfn and zap_page_range rely on CONFIG_MMU

Fix following sh-allmodconfig errors reported on kisskb
"
drivers/built-in.o: In function `ion_vm_fault':
ion.c:(.text+0x1f2d8f8): undefined reference to `vm_insert_pfn'
drivers/built-in.o: In function `ion_buffer_sync_for_device':
ion.c:(.text+0x1f316bc): undefined reference to `zap_page_range'
make: *** [vmlinux] Error 1
"

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>