fix anon-struct usage that's a warning/error -Wnon-c-typedef-for-link…

…age (#137)

Summary:
X-link: facebook/CacheLib#137

Fix
  error: anonymous non-C-compatible type given name for linkage purposes by alias declaration; add a tag name here [-Werror,-Wnon-c-typedef-for-linkage]

Reviewed By: philippv

Differential Revision: D36043476

fbshipit-source-id: c4b0203083c496c1fac4106fb500e553f3d4b5b4

fbzmq/zmq/SocketMonitor.h

@@ -88,7 +88,7 @@ class SocketMonitor {
   /**
    * event object passed down the PAIR socket to monitor class
    */
-  using EventT = struct {
+  struct EventT {
     uint16_t event;
     int32_t data;
   };