Attach zpool_influxdb to OpenZFS build

[mm]

Summary

This attaches /usr/libexec/zfs/zpool_influxdb to OpenZFS build.

OpenZFS targets this binary into "$(DESTDIR)$(zfsexecdir)" where "zfsexecdir = ${exec_prefix}/libexec/zfs"

Makefile.depend should be like zpool, how to autogenerate this file?

Test Plan

make universe passed

[freqlabs]

This looks good to me. I recall that Makefile.depend will be generated by whoever needs it and does not need to be committed if you aren't sure how to do it, but I don't see that in the committer's guide so I'm not sure where that idea came from. Hopefully someone else has input on that aspect.

[imp]

I always have to ask sjg. Maybe add him to the review?