Workaround class loader errors by loading single guava in the root scope

Possibly related to gradle/gradle#4823 as it only appeared after
using evaluationDependsOn (via The Preprocessor and on jGui).

build.gradle

@@ -1,3 +1,13 @@
+buildscript {
+    repositories {
+        mavenCentral()
+    }
+
+    dependencies {
+        classpath 'com.google.guava:guava:27.1-jre'
+    }
+}
+
 apply plugin: 'eclipse'
 apply plugin: 'idea'