sys/modules: fix standalone build for multiple build

[aokblast]

Summary

Multiple Makefile miss opt_*.h and *_if.h header file. We fix it by
running `make` in sys/modules to build all modules.

Sponsored by: The FreeBSD Foundation

[aokblast]

formatting

[lwhsu]

I believe you mean “multiple modules”

[markj]

How exactly did you test it? Remove the object directory first and then make sys/modules?

In D52062#1189371, @lwhsu wrote:

I believe you mean “multiple modules”

+1