iicbb: Always build ofw_bus_if.h

Always make ofw_bus_if.h. While it's only used when option FDT is in the
kernel, it can always be generated. In theory we could omit it if option
FDT isn't present, but none of the rest of sys/modules does that. That
fine-grained control likely won't be reliable w/o a redesign of the
kernel/module config system.

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