build: add support for SLE15SP4

[pstorz]

This PR is about adding SUSE SLE 15 SP4 platform.

Please check

• Short description and the purpose of this PR is present above this paragraph
• Your name is present in the AUTHORS file (optional)

If you have any questions or problems, please give a comment in the PR.

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• Git Workflow
• Automatic Sourcecode Formatting
• Check your commit messages

Checklist for the reviewer of the PR (will be processed by the Bareos team)

General

• PR name is meaningful
• Purpose of the PR is understood
• Separate commit for this PR in the CHANGELOG.md, PR number referenced is same
• Commit descriptions are understandable and well formatted
• [ ] If backport: add original PR number and target branch at top of this file: Backport of PR#000 to bareos-2x

Source code quality

• Source code changes are understandable
• Variable and function names are meaningful
• Code comments are correct (logically and spelling)
• Required documentation changes are present and part of the PR
• bareos-check-sources --since-merge does not report any problems
• git status should not report modifications in the source tree after building and testing

[bruno-at-bareos]

Reviewed and builded rpm tested ok.
Ready for merge