sonar-project.properties: exclude all sources from coverage metrics

Add `sonar.coverage.exclusions` to suppress SonarCloud coverage
warnings since the repo has no unit tests or coverage reports.

Signed-off-by: Simen S. Røstad <simen.rostad@nordicsemi.no>

Author: simensrostad

sonar-project.properties

@@ -3,3 +3,7 @@ sonar.organization=nrfconnect
 sonar.host.url=https://sonarcloud.io
 sonar.sources=.
 sonar.inclusions=applications/**/src/**/*.c,applications/**/src/**/*.h,lib/include/**/*.h
+
+# This repo has no unit tests or coverage reports. Exclude all sources from
+# coverage metrics so SonarCloud does not expect coverage data.
+sonar.coverage.exclusions=**/*