add checkstyle plugin to build.gradle.kts

Author: snevg04

app/build.gradle.kts

@@ -2,6 +2,7 @@ plugins {
     id("java")
     id("com.github.ben-manes.versions") version "0.51.0"
     id("org.sonarqube") version "7.1.0.6387"
+    checkstyle
     application
 }