fix: do not build modbus on openbsd

[powersj]

This is the remaining plugin that is blocking builds on OpenBSD. The issue is the github.com/grid-x/serial library does not build on OpenBSD. That library is an indirect import, due to github.com/grid-x/modbus. Upgrading modbus does not help the situation as it does not pull in a newer version of the grid-x/serial.

Because this is the only one, I figured it is simpler to stop building this on openbsd until someone comes by with a better solution.

Required for all PRs:

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolves #10034

[telegraf-tiger]

😄 This pull request doesn't change the Telegraf binary size

📦 Looks like new artifacts were built from this PR.

Expand this list to get them here! 🐯

Artifact URLs

DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	freebsd_amd64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_armv7.tar.gz
armhf.deb	i386.rpm	freebsd_i386.tar.gz
i386.deb	ppc64le.rpm	linux_amd64.tar.gz
mips.deb	s390x.rpm	linux_arm64.tar.gz
mipsel.deb	x86_64.rpm	linux_armel.tar.gz
ppc64el.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz

[srebhan]

Thanks @powersj for the fix. Code looks good even though I would prefer fixing the underlying library to provide modbus support for OpenBSD... :-)

[geowiwi]

OpenBSD support in the underlying library was fixed in:

grid-x/serial#5

Associated pull request: #10385