Add dagger dependency to plugins (again).

Keeping the compiler in the build group prevents it being put on
the classpath twice and causing massive problems.

Change-Id: I86df288f6756f4b97685ec3d4bdde2ef4633f12c
Reviewed-on: http://gerrit.dmdirc.com/2850
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager