[libc++] Fix the header guard from _LIBCPP_STEAMBUF to _LIBCPP_STREAMBUF.

GitOrigin-RevId: 27e5bffbf62cb13638102c7d3de1f386e39f0373
diff --git a/include/streambuf b/include/streambuf
index 0dfa314..6cdbdb7 100644
--- a/include/streambuf
+++ b/include/streambuf
@@ -7,8 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef _LIBCPP_STEAMBUF
-#define _LIBCPP_STEAMBUF
+#ifndef _LIBCPP_STREAMBUF
+#define _LIBCPP_STREAMBUF
 
 /*
     streambuf synopsis
@@ -495,4 +495,4 @@
 
 _LIBCPP_POP_MACROS
 
-#endif  // _LIBCPP_STEAMBUF
+#endif  // _LIBCPP_STREAMBUF