fix link error on gcc 10

Summary:
Building Folly on Fedora 32 fails with a link error, saying
SingletonVault::kDefaultShutdownTimeout_ is undefined. I did not
investigate, but this constant is only used in one place, so inline
it.

Reviewed By: yfeldblum

Differential Revision: D22713828

fbshipit-source-id: 707797eaef4633e642d07b2b02d9f55187afb99f

folly/Singleton.cpp

@@ -40,8 +40,6 @@
 
 namespace folly {
 
-constexpr std::chrono::seconds SingletonVault::kDefaultShutdownTimeout_;
-
 #if FOLLY_SINGLETON_HAVE_DLSYM
 namespace detail {
 static void singleton_hs_init_weak(int* argc, char** argv[])

folly/Singleton.h

@@ -572,9 +572,7 @@ class SingletonVault {
   Type type_;
 
   std::atomic<bool> shutdownTimerStarted_{false};
-  static constexpr std::chrono::seconds kDefaultShutdownTimeout_{
-      std::chrono::seconds{5 * 60}};
-  std::chrono::milliseconds shutdownTimeout_{kDefaultShutdownTimeout_};
+  std::chrono::milliseconds shutdownTimeout_{std::chrono::minutes{5}};
   Synchronized<std::vector<std::string>> shutdownLog_;
 };