travis: Use two threads to analyze C code with SonarQube

On Nov 12, the scanner was updated and now takes a lot more time (about
3 times as much).  Using two threads reduces it a bit (by about 25%).
Using even more threads doesn't help or even increases the time again.

scripts/test.sh

@@ -284,6 +284,7 @@ sonarcloud)
 		-Dsonar.projectKey=strongswan \
 		-Dsonar.projectVersion=$(git describe)+${TRAVIS_BUILD_NUMBER} \
 		-Dsonar.sources=. \
+		-Dsonar.cfamily.threads=2 \
 		-Dsonar.cfamily.build-wrapper-output=bw-output || exit $?
 	;;
 *)