[eclipse/xtext#1280] made genManifest / genSourcesManifest gradle 5 ready
[eclipse/xtext#1280] made genManifest / genSourcesManifest gradle 5 ready

Signed-off-by: Christian Dietrich <667c88ac3556c8ac174202176ae9c5b02bafe1e7@itemis.de>

miklossy/xtext-core,miklossy/xtext-core