build.gradle: Add clean task to root project

Missed this when restructuring

Signed-off-by: Álvaro Brey Vilas <alvaro.brey@nextcloud.com>

build.gradle

@@ -37,4 +37,8 @@ subprojects {
         maven { url "https://jitpack.io" }
         mavenCentral()
     }
+}
+
+task clean(type: Delete) {
+    delete rootProject.buildDir
 }