[libc++] Alphabetize and include-what-you-use. NFCI.

Differential Revision: https://reviews.llvm.org/D102781

NOKEYCHECK=True
GitOrigin-RevId: bfbd73f87d7b2c1140b43192cea6b7fbbfc35cd0
diff --git a/include/streambuf b/include/streambuf
index ee1afac..385d486 100644
--- a/include/streambuf
+++ b/include/streambuf
@@ -108,8 +108,8 @@
 */
 
 #include <__config>
-#include <iosfwd>
 #include <ios>
+#include <iosfwd>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header