Missing public base class for unicode_error

Summary: [Folly] Missing public base class for `unicode_error`. which should publicly inherit `std::runtime_error`.

Reviewed By: Mizuchi

Differential Revision: D22153547

fbshipit-source-id: 4c60e8e072f0fc9240d199b6d81c7e85c99eaf34

folly/Unicode.h

@@ -26,7 +26,7 @@
 
 namespace folly {
 
-class FOLLY_EXPORT unicode_error : std::runtime_error {
+class FOLLY_EXPORT unicode_error : public std::runtime_error {
  public:
   using std::runtime_error::runtime_error;
 };