[J10] Resolve illegal reflective access in MergeableManifest

[kthoms]

The following warnings are reported when running an Xtext generator with Java 10:

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.eclipse.xtext.util.MergeableManifest (file:/[...]/xtext-core/org.eclipse.xtext.util/bin/) to field java.util.jar.Manifest.attr
WARNING: Please consider reporting this to the maintainers of org.eclipse.xtext.util.MergeableManifest
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

[cdietrich]

There is a dupe for that somewhere in the bunch

[kthoms]

Ok then we‘ll close this one

[cdietrich]

see #506