[libc++][NFC] Remove unused includes from poll_with_backoff.h

Differential Revision: https://reviews.llvm.org/D138687

NOKEYCHECK=True
GitOrigin-RevId: 732bf92c9e072b72196cef0d0137b3eb73042f80
diff --git a/include/module.modulemap.in b/include/module.modulemap.in
index 5f07d9d..1a33ec2 100644
--- a/include/module.modulemap.in
+++ b/include/module.modulemap.in
@@ -672,6 +672,7 @@
         private header "__chrono/high_resolution_clock.h"
         export steady_clock
         export system_clock
+        export time_point
       }
       module literals               { private header "__chrono/literals.h" }
       module month                  { private header "__chrono/month.h" }