Remove extraneous semicolon from proxygen/lib/http/HTTPHeaders.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: meyering

Differential Revision: D51631140

fbshipit-source-id: 3ae9c699ea6eca3e110e9ddd23843c0fd922fcb7

proxygen/lib/http/HTTPHeaders.h

@@ -453,7 +453,8 @@ void HTTPHeaders::add(HTTPHeaderCode code, T&& value) {
       const size_t pos = ptr - codes();                   \
       {Block} ptr++;                                      \
     }                                                     \
-  }
+  }                                                       \
+  static_assert(true, "semicolon required")
 
 // iterate over the positions of all headers with given name
 #define ITERATE_OVER_STRINGS(String, Block)              \