Add staging repo in one more place in Gradle build to make compile succeed on a fresh machine

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1838729 13f79535-47bb-0310-9956-ffa450edef68

build.gradle

@@ -104,6 +104,7 @@ subprojects {
     sourceCompatibility = 1.8
 
     repositories {
+        maven { url "https://repository.apache.org/content/repositories/staging" }
         mavenCentral()
     }