Expose gloop peer friendship for testing

PiperOrigin-RevId: 895336821
Change-Id: I178e7d654e45de4af6bba133b496115c6d37adc1

absl/synchronization/internal/per_thread_sem.h

diff --git a/absl/synchronization/internal/per_thread_sem.h b/absl/synchronization/internal/per_thread_sem.h
index 144ab3c..704f3da 100644
--- a/absl/synchronization/internal/per_thread_sem.h
+++ b/absl/synchronization/internal/per_thread_sem.h
@@ -31,6 +31,10 @@
 #include "absl/synchronization/internal/create_thread_identity.h"
 #include "absl/synchronization/internal/kernel_timeout.h"
 
+namespace gloop_do_not_use {
+struct SynchronizationBenchmarkPeer;
+}  // namespace gloop_do_not_use
+
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
@@ -77,6 +81,7 @@
   // Permitted callers.
   friend class PerThreadSemTest;
   friend class absl::Mutex;
+  friend struct ::gloop_do_not_use::SynchronizationBenchmarkPeer;
   friend void OneTimeInitThreadIdentity(absl::base_internal::ThreadIdentity*);
 };