Contribute generated SRG files to ForgeGradle's source remapping task

[stephan-gh]

As of ForgeGradle 2 it is now reobfuscating the sources to SEARGE as well, so they can be remapped to the local MCP mapping version using the local dependency deobfuscation similar to using the obfuscated JAR as dependency in the dev environment.

Together with SpongePowered/Mixin#91 it would be good if we would not only contribute the Mixin SRGs to the reobfuscation task, but also to the source remapper so the mixins sources get reobfuscated properly. There is a remapping task for each of the source sets so the SRG file needs to be added to all of them.