Fix -Wstrict-prototypes warning in thread_pthread.h. (GH-21477)

(cherry picked from commit ea62a4bd54421693ed6b24a1bbd18ebed3bdb8f8)

Co-authored-by: Benjamin Peterson <benjamin@python.org>
diff --git a/Python/thread_pthread.h b/Python/thread_pthread.h
index cf4e854..e6910b3 100644
--- a/Python/thread_pthread.h
+++ b/Python/thread_pthread.h
@@ -134,7 +134,7 @@
 static pthread_condattr_t *condattr_monotonic = NULL;
 
 static void
-init_condattr()
+init_condattr(void)
 {
 #ifdef CONDATTR_MONOTONIC
     static pthread_condattr_t ca;