Add required repositories block to samples

Starting with Kotlin 1.2.60.*, afaict, the Kotlin plugin downloads the
Kotlin compiler and its dependencies on demand using a project
configuration, which requires a repository to be configured. These
repositories were not necessary until the upgrade to Kotlin 1.2.60
because the kotlin-eap repository was being automatically added to
every project (to ease migration).

samples/buildSrc-plugin/buildSrc/build.gradle.kts

@@ -11,3 +11,7 @@ gradlePlugin {
         }
     }
 }
+
+repositories {
+    jcenter()
+}

samples/gradle-plugin/plugin/build.gradle.kts

@@ -21,3 +21,7 @@ publishing {
         maven(url = "build/repository")
     }
 }
+
+repositories {
+    jcenter()
+}

samples/model-rules/buildSrc/build.gradle.kts

@@ -1,3 +1,7 @@
 plugins {
     `kotlin-dsl`
 }
+
+repositories {
+    jcenter()
+}

samples/multi-kotlin-project-with-buildSrc/buildSrc/build.gradle.kts

@@ -1,3 +1,7 @@
 plugins {
     `kotlin-dsl`
 }
+
+repositories {
+    jcenter()
+}

samples/multi-project-with-buildSrc/buildSrc/build.gradle.kts

@@ -1,3 +1,7 @@
 plugins {
     `kotlin-dsl`
 }
+
+repositories {
+    jcenter()
+}

samples/precompiled-script-plugin/plugin/build.gradle.kts

@@ -15,3 +15,7 @@ publishing {
         maven(url = "build/repository")
     }
 }
+
+repositories {
+    jcenter()
+}

samples/project-with-buildSrc/buildSrc/build.gradle.kts

@@ -1,3 +1,7 @@
 plugins {
     `kotlin-dsl`
 }
+
+repositories {
+    jcenter()
+}