recovery: Add the missing #include.

Although I checked there's no reference to 'stage' in device-specific
recovery codes (for commit a8d72bc3),
it's insufficient to capture the missing #include issue.

Test: lunch aosp_dragon-userdebug; mmma bootable/recovery
Change-Id: I4c5dbe9acf38918491c3aa776ce358be03f9ecca