flb_utils.c: fix for macOS < 12

[barracuda156]

kIOMainPortDefault is available on macOS 12+: https://developer.apple.com/documentation/iokit/kiomainportdefault
Add a fallback definition to fix compilation on earlier systems.

Verified to work on macOS 10.6.

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[cosmo0920]

Yes, this is one of the candidates to fix compilation issue that is caused by an absence of kIOMainPortDefault on macOS 11.0 or earlier.

[edsiper]

thanks for the contribution. for commits just need to fix the prefix (I will do it for now)

flb_utils.c: fix for macOS < 12

• use prefix utils: ...
• be more descriptive on what's the fix

[barracuda156]

thanks for the contribution. for commits just need to fix the prefix (I will do it for now)

flb_utils.c: fix for macOS < 12

• use prefix utils: ...

• be more descriptive on what's the fix

@edsiper Noted. Thank you!