Remove extraneous semicolon from folly/synchronization/HazptrHolder.h

Summary:
Extraneous semicolons are a code smell and can mask more serious problems. `-Wextra-semi-stmt` finds them.

This diff removes an extraneous semicolon or adjusts a macro so that a semicolon is required after the macro (making it look like a standard function).

This file is drawn from a heavy-hitting list, so fixing this problem will allow *many* other files to take advantage of the safety `-Wextra-semi-stmt` offers.

This should be a low-risk diff: if it compiles, it works.

Reviewed By: Orvid

Differential Revision: D51631113

fbshipit-source-id: cb26e946c075a9b45d11a6c1d47a6b523188c407

folly/synchronization/HazptrHolder.h

@@ -139,7 +139,7 @@ class hazptr_holder {
   FOLLY_ALWAYS_INLINE T* protect(const Atom<T*>& src, Func f) noexcept {
     T* ptr = src.load(std::memory_order_relaxed);
     while (!try_protect(ptr, src, f)) {
-      /* Keep trying */;
+      /* Keep trying */
     }
     return ptr;
   }