folly/experimental/fibers/EventBaseLoopController.h: include <atomic>

Summary: Without <atomic>, this fails to compile with clang:dev and newer glibc.

Test Plan:
fbconfig --clang --with-project-version=clang:dev --with-project-version=llvm_compiler_rt:dev --platform-all=gcc-4.9-glibc-2.20 --sanitize=address mcrouter/lib/network:client-test-sync && fbmake dbg

Reviewed By: pavlo@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant

FB internal diff: D2093709

Signature: t1:2093709:1432250378:2b78c142598bf0a7b0869089c752f5ccdae9da22

folly/experimental/fibers/EventBaseLoopController.h

@@ -16,6 +16,7 @@
 #pragma once
 
 #include <memory>
+#include <atomic>
 #include <folly/experimental/fibers/LoopController.h>
 
 namespace folly {