Add netbsd an openbsd to build tags of process_common.go

[kvch]

What does this PR do?

This PR adds openbsd and freebsd to the list of build tags protecting process_common.go file.

Why is it important?

Without this change libbeat cannot be crosscompiled on openbsd and freebsd:

2 errors occurred:
 --> openbsd/amd64 error: exit status 2
 Stderr: # github.com/elastic/elastic-agent-system-metrics/metric/system/process
 ../../../../../pkg/mod/github.com/elastic/elastic-agent-system-metrics@v0.3.0/metric/system/process/process_types.go:32:11: undefined: PidState
 ../../../../../pkg/mod/github.com/elastic/elastic-agent-system-metrics@v0.3.0/metric/system/process/process_types.go:161:10: undefined: PidState

 --> netbsd/amd64 error: exit status 2
 Stderr: # github.com/elastic/elastic-agent-system-metrics/metric/system/process
 ../../../../../pkg/mod/github.com/elastic/elastic-agent-system-metrics@v0.3.0/metric/system/process/process_types.go:32:11: undefined: PidState
 ../../../../../pkg/mod/github.com/elastic/elastic-agent-system-metrics@v0.3.0/metric/system/process/process_types.go:161:10: undefined: PidState

Checklist

• My code follows the style guidelines of this project
  - [ ] I have commented my code, particularly in hard-to-understand areas
  - [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md

Related issues

Required by elastic/beats#31615

[elasticmachine]

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• Start Time: 2022-05-20T15:00:20.009+0000

• Duration: 11 min 49 sec

Test stats 🧪

Test	Results
Failed	0
Passed	93
Skipped	2
Total	95

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