gerrit-plugin-api: Export gson as a dependency

So it can be used in plugins without them having to define it
themselves (potentially with a different version).

Change-Id: Ibb15acdd75640cd18b21ad5ac5895fd164cebccd

gerrit-plugin-api/BUCK

@@ -29,6 +29,7 @@ java_library(
     '//gerrit-reviewdb:server',
     '//lib:args4j',
     '//lib:blame-cache',
+    '//lib:gson',
     '//lib:guava',
     '//lib:gwtorm',
     '//lib:jsch',