iiod: Add default env var to service file

[dennis-ju]

Systemd now gives a warning when IIOD_EXTRA_OPTS is unset.

This change sets it to an empty string by default while still allowing it be overridden by /etc/default/iiod.

Fixes #1142.

[nunojsa]

Just one question.... Does this also affects the main branch? If so, the natural flow is to change the target branch to main (in this PR) and then cherry pick this into libiio-v0

Forget about this... just saw that the commit is already in main. Next time run cherry-pick with -x and then it's clear to me :)

[cristina-suteu]

can I merge this ?

[nunojsa]

can I merge this ?

@mhennerich ?

[dennis-ju]

Just one question.... Does this also affects the main branch? If so, the natural flow is to change the target branch to main (in this PR) and then cherry pick this into libiio-v0

Forget about this... just saw that the commit is already in main. Next time run cherry-pick with -x and then it's clear to me :)

Alright, I'll do that next time :)