Fix OSS build with gcc 4.9
Fix OSS build with gcc 4.9

Summary: This static initalizer implicitly invokes an explicit
constructor, but apparently only under gcc 4.9 (gcc 4.8 either doesn't
notice or has a different libstdc++ or something). Fix the build error.

Ref. https://github.com/facebook/hhvm/issues/5438

Test Plan: Builds on an Ubuntu 15.04 machine (gcc 4.9.2). Still builds
in fbcode (I don't think this file is used in fbcode though).

Reviewed By: @pavlo-fb

Differential Revision: D2118922
reddit/mcrouter,tempbottle/mcrouter,zhlong73/mcrouter,nvaller/mcrouter,reddit/mcrouter,glensc/mcrouter,reddit/mcrouter,reddit/mcrouter,facebook/mcrouter,glensc/mcrouter,is00hcw/mcrouter,facebook/mcrouter,easyfmxu/mcrouter,tempbottle/mcrouter,nvaller/mcrouter,facebook/mcrouter,glensc/mcrouter,easyfmxu/mcrouter,is00hcw/mcrouter,easyfmxu/mcrouter,evertrue/mcrouter,nvaller/mcrouter,tempbottle/mcrouter,yqzhang/mcrouter,yqzhang/mcrouter,leitao/mcrouter,easyfmxu/mcrouter,tempbottle/mcrouter,evertrue/mcrouter,zhlong73/mcrouter,is00hcw/mcrouter,nvaller/mcrouter,synecdoche/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,leitao/mcrouter,yqzhang/mcrouter,synecdoche/mcrouter,zhlong73/mcrouter,facebook/mcrouter,is00hcw/mcrouter,leitao/mcrouter,glensc/mcrouter,zhlong73/mcrouter,evertrue/mcrouter,synecdoche/mcrouter,evertrue/mcrouter,leitao/mcrouter