Fix undeclared task dependency in configureReobfTask setup

Without it, it may run ahead of the compile task, where the tsrgFile is
missing, and the configuration will not be properly applied

Fixes SpongePowered#38

src/main/groovy/org/spongepowered/asm/gradle/plugins/MixinExtension.groovy

@@ -1052,6 +1052,8 @@ public class MixinExtension {
                 def configureReobfTaskTask = project.tasks.maybeCreate("configureReobfTaskFor${reobfTask.name.capitalize()}", configureReobfTaskType).configure {
                     delegate.reobfTask = reobfTask
                     mappingFiles += tsrgFile
+                    // tsrgFile is generated by compileTask
+                    dependsOn(compileTask)
                 }
                 reobfTask.dependsOn configureReobfTaskTask
             }