Fix exclusive_owner_ spelling

... to fix Mac build breakage.

Test: Grep for exclusive_owner_ misspellings + TreeHugger

Bug: 65171052
Change-Id: I15eec0651b2d6d8ce20a8a906c5943c0f1aaa7fe
diff --git a/runtime/base/mutex.cc b/runtime/base/mutex.cc
index d1ea13a..008880c 100644
--- a/runtime/base/mutex.cc
+++ b/runtime/base/mutex.cc
@@ -955,7 +955,7 @@
   guard_.num_contenders_--;
 #else
   pid_t old_owner = guard_.GetExclusiveOwnerTid();
-  guard_.exclusive_owner.StoreRelaxed(0);
+  guard_.exclusive_owner_.StoreRelaxed(0);
   guard_.recursion_count_ = 0;
   CHECK_MUTEX_CALL(pthread_cond_wait, (&cond_, &guard_.mutex_));
   guard_.exclusive_owner_.StoreRelaxed(old_owner);