Use decode_xml_wineventlog in system Splunk inputs

[marc-gr]

What does this PR do?

Uses decode_xml_wineventlog processor instead of decode_xml

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.

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Build Cause: Branch indexing

• Start Time: 2021-04-21T22:28:16.997+0000

• Duration: 46 min 43 sec

• Commit: 9b2ae55

Test stats 🧪

Test	Results
Failed	0
Passed	1926
Skipped	3
Total	1929

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[leehinman]

LGTM

[marc-gr]

/test