Mark a couple of constexpr values as static so they don't need to be captured

Summary: Because if you tried to capture them, they wouldn't be usable as template arguments.

Reviewed By: yfeldblum

Differential Revision: D9565384

fbshipit-source-id: 9c4579848be1c5c48251c17b0f55443aade75eba

folly/futures/test/WindowTest.cpp

@@ -142,9 +142,9 @@ TEST(Window, exception) {
 
 TEST(Window, stackOverflow) {
   // Number of futures to spawn.
-  constexpr size_t m = 1000;
+  static constexpr size_t m = 1000;
   // Size of each block of input and output.
-  constexpr size_t n = 1000;
+  static constexpr size_t n = 1000;
 
   std::vector<std::array<int, n>> ints;
   int64_t expectedSum = 0;