Fix FBString.h inclusion order

Summary: I don't see how this header works, as it includes folly/Portable.h (which defines FOLLY_HAS_STRING_VIEW) _after_ it has already used FOLLY_HAS_STRING_VIEW a few lines up. Somehow, that worked with buck1 (really not sure how), but broke buck2.

Reviewed By: ot

Differential Revision: D33511408

fbshipit-source-id: 9e2da2a5c25a02367079254ff78552a5a335b3b1

folly/FBString.h

@@ -19,34 +19,32 @@
 
 #pragma once
 
+#include <algorithm>
 #include <atomic>
+#include <cassert>
 #include <cstddef>
+#include <cstring>
 #include <iosfwd>
 #include <limits>
 #include <stdexcept>
+#include <string>
 #include <type_traits>
-
-#if FOLLY_HAS_STRING_VIEW
-#include <string_view>
-#endif
+#include <utility>
 
 #include <folly/CPortability.h>
 #include <folly/CppAttributes.h>
 #include <folly/Likely.h>
 #include <folly/Portability.h>
-
-#include <algorithm>
-#include <cassert>
-#include <cstring>
-#include <string>
-#include <utility>
-
 #include <folly/Traits.h>
 #include <folly/hash/Hash.h>
 #include <folly/lang/Assume.h>
 #include <folly/lang/Exception.h>
 #include <folly/memory/Malloc.h>
 
+#if FOLLY_HAS_STRING_VIEW
+#include <string_view>
+#endif
+
 FOLLY_PUSH_WARNING
 // Ignore shadowing warnings within this file, so includers can use -Wshadow.
 FOLLY_GNU_DISABLE_WARNING("-Wshadow")