fix iostat error on MINIMAL

[EvilOlaf]

Description

MINIMAL do not have iostat so suppress the error occurring when doing armbianmonitor -u on such systems.
I am pretty sure there is a nicer way to implement this. So some wizard may come up with something better.

How Has This Been Tested?

• do armbianmonitor -u on a minimal system and check if an error occurs

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[EvilOlaf]

I have no idea why I sent that fix in the middle of a meeting. This bugged me forever....

[igorpecovnik]

looks good, no need to do it nicer