JAR of sonar-xoo-plugin should not contain sonar-plugin-api

* Plugins should not embed sonar-plugin-api, unfortunately this
  happened due to typo in efad42ce25

* And this was preventing caching of build task ":sonar-scanner-engine:test"

plugins/sonar-xoo-plugin/build.gradle

@@ -7,7 +7,7 @@ dependencies {
   compile 'commons-io:commons-io'
   compile 'commons-lang:commons-lang'
   compile 'org.apache.commons:commons-csv'
-  compile project(path: ':sonar-plugin-api', configuration: 'shadow')
+  compileOnly project(path: ':sonar-plugin-api', configuration: 'shadow')
   compileOnly 'com.google.code.findbugs:jsr305'
 
   testCompile 'junit:junit'