reporter - create dirs for exported file

[stbischof]

fixes:

Caused by: org.apache.maven.plugin.MojoExecutionException: Failed to write the report at /home/stbischof/dev/git/org.eclipse.aaa.bbb/prototype/core/target/docs_a_metadata.json
    at aQute.bnd.maven.reporter.plugin.ExportMojo.execute (ExportMojo.java:132)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
    at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute (MojoExecutor.java:301)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:211)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:165)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:157)

[bjhargrave]

Thanks!