Add a dependency on the local tools.jar for doc tool generation.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=225565675

gradle/core/build.gradle

@@ -262,6 +262,9 @@ dependencies {
   // Dependencies needed for compiling stylesheets to javascript
   css 'com.google.closure-stylesheets:closure-stylesheets:1.5.0'
   css 'args4j:args4j:2.0.26'
+
+  // Tool dependencies.  used for doc generation.
+  compile files("${System.properties['java.home']}/../lib/tools.jar")
 }
 
 task jaxbToJava {