Mark groovy-eclipse-compiler mojos threadsafe

[TobiX]

This is a followup to #449 - While the fix marked the compiler component as threadSafe, the plugin still contains the AddGroovySourceFolders mojo, which is still called out by maven:

[WARNING] *****************************************************************
[WARNING] * Your build is requesting parallel execution, but project      *
[WARNING] * contains the following plugin(s) that have goals not marked   *
[WARNING] * as @threadSafe to support parallel building.                  *
[WARNING] * While this /may/ work fine, please look for plugin updates    *
[WARNING] * and/or request plugins be made thread-safe.                   *
[WARNING] * If reporting an issue, report it against the plugin in        *
[WARNING] * question, not against maven-core                              *
[WARNING] *****************************************************************
[WARNING] The following goals are not marked @threadSafe in XXX:
[WARNING] org.codehaus.groovy:groovy-eclipse-compiler:3.4.0-01:add-groovy-build-paths
[WARNING] *****************************************************************

Probably also a good time to switch to annotation-based config:

@Mojo(name = "add-groovy-build-paths", defaultPhase = LifecyclePhase.INITIALIZE, requiresProject = true, threadSafe = true)

and for the project member:

@Parameter(defaultValue = "${project}", readonly = true)