Mark a couple more local constexpr values as static

Summary: Because MSVC doesn't let you access it without capturing it from within a lambda otherwise.

Reviewed By: yfeldblum

Differential Revision: D4199867

fbshipit-source-id: 7b829b2cc373edbebe3498901fa323c89a211818

folly/io/async/test/EventBaseTest.cpp

@@ -1829,8 +1829,8 @@ TEST(EventBaseTest, LoopKeepAliveShutdown) {
 TEST(EventBaseTest, LoopKeepAliveAtomic) {
   auto evb = folly::make_unique<EventBase>();
 
-  constexpr size_t kNumThreads = 100;
-  constexpr size_t kNumTasks = 100;
+  static constexpr size_t kNumThreads = 100;
+  static constexpr size_t kNumTasks = 100;
 
   std::vector<std::thread> ts;
   std::vector<std::unique_ptr<Baton<>>> batons;