Add a friendship for gloop testing

PiperOrigin-RevId: 904670147
Change-Id: Ib9741bf43ce4eb07476585610eba31e1e4c206a9

absl/base/internal/unscaledcycleclock.h

diff --git a/absl/base/internal/unscaledcycleclock.h b/absl/base/internal/unscaledcycleclock.h
index 94c4bea..fb5751b 100644
--- a/absl/base/internal/unscaledcycleclock.h
+++ b/absl/base/internal/unscaledcycleclock.h
@@ -79,6 +79,7 @@
   friend class base_internal::CycleClock;
   friend class time_internal::UnscaledCycleClockWrapperForGetCurrentTime;
   friend class base_internal::UnscaledCycleClockWrapperForInitializeFrequency;
+  friend class gloop_do_not_use::UnscaledCycleClockWrapperForPerCpuTest;
 };
 
 #if defined(__x86_64__)