make pgen.c C89 compliant again (GH-7870)

diff --git a/Parser/pgen.c b/Parser/pgen.c
index 5968a49..6b2cdec 100644
--- a/Parser/pgen.c
+++ b/Parser/pgen.c
@@ -120,7 +120,8 @@
 static void
 freenfagrammar(nfagrammar *gr)
 {
-    for (int i = 0; i < gr->gr_nnfas; i++) {
+    int i;
+    for (i = 0; i < gr->gr_nnfas; i++) {
         PyObject_FREE(gr->gr_nfa[i]->nf_state);
     }
     PyObject_FREE(gr->gr_nfa);