Update Buck

Buck does not accept duplicate resources, which happens with
gwt_module() loading in srcs, resources and gwt_xml. Convert
to a set and back to a list to remove duplicates transparently.

Contributed-by: Shawn Pearce <sop@google.com>
Change-Id: Ic27ab2f34c60c8cb54373d93fcd0c7cac7477a3b