Mark mojom_bindings_generator rules as restat

mojom_bindings_generator only updates the output file if its
contents have changed, add restat to the rule so that ninja
checks the timestamp of the file after running the rule and
considers it clean if it didn't change.

Test: m checkbuild && m checkbuild
Change-Id: Ibc155246faa4bc3b4f6fe328904857479a949e14