[#38] Fix sonar.sources attribute in sonar-project.properties

[panagiotisbellias]

Description

Fix sonar-project.properties in order to be analyzed from sonarqube

Related Issue

• Fixes

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Tests only (no source changes)

Checklist:

• [x] Code changes
• [ ] Unit tests added
• [ ] Acceptance tests added

[OleMussmann]

Hey @panagiotisbellias , thanks for your contribution! We're still fixing some CI failures, we will get to your PR soon after.