folly::settings do not inline settings definition

Summary:
If FOLLY_SETTING_DEFINE(...) is put into .cpp file right now you will get linker error about undefined references.

According to http://en.cppreference.com/w/cpp/language/inline
  2) The definition of an inline function or variable (since C++17) must be present in the translation unit where it is accessed (not necessarily before the point of access).

Hence the linker error. This diff fixes that.

Reviewed By: alikhtarov, mnv104

Differential Revision: D7567469

fbshipit-source-id: 02049e4881e50c414aeb506745e02eed1256daad

folly/experimental/settings/Settings.h

@@ -122,7 +122,7 @@ class Setting {
  */
 #define FOLLY_SETTING_DEFINE(_project, _name, _Type, _def, _desc)         \
   /* Meyers singleton to avoid SIOF */                                    \
-  inline folly::settings::detail::Setting<_Type>&                         \
+  folly::settings::detail::Setting<_Type>&                                \
       FOLLY_SETTINGS_FUNC__##_project##_##_name() {                       \
     static folly::Indestructible<folly::settings::detail::Setting<_Type>> \
         setting(                                                          \