[libc][NFC] Add a check to catch mismatch in internal and public mutex types.

GitOrigin-RevId: f7e572b4f4a0339162f8bc168edfd9fa523c9c1e
diff --git a/src/threads/linux/Mutex.h b/src/threads/linux/Mutex.h
index 454b8ce..f6033c9 100644
--- a/src/threads/linux/Mutex.h
+++ b/src/threads/linux/Mutex.h
@@ -107,6 +107,10 @@
   }
 };
 
+static_assert(sizeof(Mutex) == sizeof(mtx_t),
+              "Sizes of internal representation of mutex and the public mtx_t "
+              "do not match.");
+
 class MutexLock {
   Mutex *mutex;