update build script (add buildSrc)

buildSrc/build.gradle.kts

@@ -0,0 +1,11 @@
+plugins {
+    // Support convention plugins written in Kotlin. Convention plugins are build scripts in 'src/main' that automatically become available as plugins in the main build.
+    `kotlin-dsl`
+}
+
+repositories {
+    // Use the plugin portal to apply community plugins in convention plugins.
+    gradlePluginPortal()
+}
+dependencies {
+}