Make lemming buildable on non-Linux envs

[DanG100]

This is just a quick fix, I'd like to take a deeper look on the possibility of still being to run the dataplane on not linux

[wenovus]

Thanks this looks neat that we're able to separate out the linux-only interface manager such that it's not compiled unless we're on a linux machine.

[github-actions]

Pull Request Test Coverage Report for Build 5872545144

• 0 of 28 (0.0%) changed or added relevant lines in 5 files are covered.
• 11 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.002%) to 2.499%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dataplane/internal/engine/engine.go	0	1	0.0%
dataplane/server.go	0	1	0.0%
dataplane/reconcilers_linux.go	0	3	0.0%
lemming.go	0	3	0.0%
dataplane/internal/engine/engine_linux.go	0	20	0.0%

Files with Coverage Reduction	New Missed Lines	%
gnmi/oc/networkinstance/networkinstance-0.go	3	0.27%
dataplane/internal/kernel/kerneltest/kernel.go	8	72.6%

Totals
Change from base Build 5857578806:	-0.002%
Covered Lines:	13212
Relevant Lines:	528743

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💛 - Coveralls