[libc++] Remove unused <iosfwd> include from <__debug>

Differential Revision: https://reviews.llvm.org/D122999

NOKEYCHECK=True
GitOrigin-RevId: e70533ae6c57756111689abf7826a3c632255866
diff --git a/include/module.modulemap b/include/module.modulemap
index 75e9413..10e944a 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -648,7 +648,10 @@
       module move_iterator         { private header "__iterator/move_iterator.h" }
       module next                  { private header "__iterator/next.h" }
       module ostream_iterator      { private header "__iterator/ostream_iterator.h" }
-      module ostreambuf_iterator   { private header "__iterator/ostreambuf_iterator.h" }
+      module ostreambuf_iterator   {
+        private header "__iterator/ostreambuf_iterator.h"
+        export iosfwd
+      }
       module permutable            { private header "__iterator/permutable.h" }
       module prev                  { private header "__iterator/prev.h" }
       module projected             { private header "__iterator/projected.h" }