Add missed rootProject for Checkstyle location

The Checkstyle plugin is applied in every sub-project
and Gradle `file()` function uses that sub-project absolute
path if the path provided is relative

* Add a `rootProject` reference for `file()` function
to ensure that we obtain Checkstyle config location
exactly from the root project

Author: artembilan

build.gradle

@@ -176,7 +176,7 @@ subprojects { subproject ->
 	}
 
 	checkstyle {
-		configDirectory.set(file("src/checkstyle"))
+		configDirectory.set(rootProject.file("src/checkstyle"))
 		toolVersion = '8.24'
 	}