[system] fix adding additional processors in syslog data stream

[kaiyan-sheng]

What does this PR do?

Similar to #4395, this PR is to fix adding processors for syslog in system package.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

[elasticmachine]

🚀 Benchmarks report

Package system 👍(2) 💚(0) 💔(1)

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Data stream	Previous EPS	New EPS	Diff (%)	Result
syslog	14705.88	11627.91	-3077.97 (-20.93%)	💔

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[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2022-10-05T20:46:25.145+0000

• Duration: 15 min 48 sec

Test stats 🧪

Test	Results
Failed	0
Passed	246
Skipped	0
Total	246

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (3/3)	💚
Files	100.0% (4/4)	💚 2.508
Classes	100.0% (4/4)	💚 2.508
Methods	60.759% (48/79)	👎 -29.53
Lines	98.793% (2702/2735)	👍 7.158
Conditionals	100.0% (0/0)	💚