bpo-36346: Make unicodeobject.h C89 compatible (GH-20934)

diff --git a/Include/cpython/unicodeobject.h b/Include/cpython/unicodeobject.h
index 569bdb1..7e53ccc 100644
--- a/Include/cpython/unicodeobject.h
+++ b/Include/cpython/unicodeobject.h
@@ -53,7 +53,8 @@
 
 Py_DEPRECATED(3.3) static inline void
 Py_UNICODE_FILL(Py_UNICODE *target, Py_UNICODE value, Py_ssize_t length) {
-    for (Py_ssize_t i = 0; i < length; i++) {
+    Py_ssize_t i;
+    for (i = 0; i < length; i++) {
         target[i] = value;
     }
 }