finagle/finagle-mux: Add BUILD export for finagle-core for symbol com…

….twitter.finagle.ssl.OpportunisticTls.Off and others

Problem
Bazel builds depending on finagle-mux fail because unless they also depend on
finagle-core, because using symbol `com.twitter.finagle.mux.transport.OpportunisticTls.Off` from
finagle-mux requires having symbol `com.twitter.finagle.ssl.OpportunisticTls.Off` from finagle-core
available.

Solution
Adds an explicit export of finagle-core to the finagle-mux BUILD

Differential Revision: https://phabricator.twitter.biz/D628678

finagle-mux/src/main/scala/BUILD

@@ -21,5 +21,6 @@ scala_library(
     ],
     exports = [
         "3rdparty/jvm/io/netty:netty4",
+        "finagle/finagle-core",
     ],
 )