use `const` in graminit.c (GH-12713)

diff --git a/Parser/pgen/grammar.py b/Parser/pgen/grammar.py
index 1ab9434..5cd6524 100644
--- a/Parser/pgen/grammar.py
+++ b/Parser/pgen/grammar.py
@@ -76,7 +76,7 @@
 
     def print_labels(self, writer):
         writer(
-            "static label labels[{n_labels}] = {{\n".format(n_labels=len(self.labels))
+            "static const label labels[{n_labels}] = {{\n".format(n_labels=len(self.labels))
         )
         for label, name in self.labels:
             label_name = '"{}"'.format(name) if name is not None else 0
@@ -89,7 +89,7 @@
 
     def print_dfas(self, writer):
         self.print_states(writer)
-        writer("static dfa dfas[{}] = {{\n".format(len(self.dfas)))
+        writer("static const dfa dfas[{}] = {{\n".format(len(self.dfas)))
         for dfaindex, dfa_elem in enumerate(self.dfas.items()):
             symbol, (dfa, first_sets) = dfa_elem
             writer(
@@ -131,7 +131,7 @@
         for stateindex, state in enumerate(states):
             narcs = len(state)
             write(
-                "static arc arcs_{dfa_index}_{state_index}[{n_arcs}] = {{\n".format(
+                "static const arc arcs_{dfa_index}_{state_index}[{n_arcs}] = {{\n".format(
                     dfa_index=dfaindex, state_index=stateindex, n_arcs=narcs
                 )
             )