zfs: attach zpool_influxdb to build

From the zpool_influxdb.8 manual page:
zpool_influxdb produces InfluxDB-line-protocol-compatible metrics from
zpools. Like the zpool command, zpool_influxdb

zfs: attach zpool_influxdb to build

From the zpool_influxdb.8 manual page:
zpool_influxdb produces InfluxDB-line-protocol-compatible metrics from
zpools. Like the zpool command, zpool_influxdb reads the current pool
status and statistics. Unlike the zpool command which is intended for
humans, zpool_influxdb formats the output in the InfluxDB line protocol.
The expected use is as a plugin to a metrics collector or aggregator,
such as Telegraf.

zpool_influxdb is installed into /usr/libexec/zfs/

Differential revision: https://reviews.freebsd.org/D31094

(cherry picked from commit 48b4fe0503282f03d25e23f44109c5cb6d450f7c)

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