Canonical implementation of IoUringBackend::NotAvailable

Summary: [Folly] Canonical implementation of `IoUringBackend::NotAvailable` - need to export exceptions since they are the most likely use-case for RTTI and can just bring the base class's ctors into scope.

Differential Revision: D19154080

fbshipit-source-id: efaf9440a5ec3d78201448160efe4b8893314956

folly/experimental/io/IoUringBackend.h

@@ -27,10 +27,9 @@ namespace folly {
 
 class IoUringBackend : public PollIoBackend {
  public:
-  class NotAvailable : public std::runtime_error {
+  class FOLLY_EXPORT NotAvailable : public std::runtime_error {
    public:
-    explicit NotAvailable(const std::string& what) noexcept
-        : std::runtime_error(what) {}
+    using std::runtime_error::runtime_error;
   };
 
   explicit IoUringBackend(