Enabled incremental compilation by default in Gradle plugin.

#KT-16546 Fixed

(cherry picked from commit 06715c5)

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/tasks/Tasks.kt

@@ -86,6 +86,10 @@ abstract class AbstractKotlinCompile<T : CommonCompilerArguments>() : AbstractCo
             System.setProperty("kotlin.incremental.compilation.experimental", value.toString())
         }
 
+    init {
+        incremental = true //to execute the setter as well
+    }
+
     internal var coroutinesFromGradleProperties: Coroutines? = null
     // Input is needed to force rebuild even if source files are not changed
     @get:Input