py: Declare constant data as properly constant.

Otherwise some compilers (eg without optimisation) will put this read-only
data in RAM instead of ROM.
diff --git a/py/compile.c b/py/compile.c
index d40d8a1..df6dab0 100644
--- a/py/compile.c
+++ b/py/compile.c
@@ -2649,7 +2649,7 @@
 }
 
 typedef void (*compile_function_t)(compiler_t*, mp_parse_node_struct_t*);
-STATIC compile_function_t compile_function[] = {
+STATIC const compile_function_t compile_function[] = {
 #define nc NULL
 #define c(f) compile_##f
 #define DEF_RULE(rule, comp, kind, ...) comp,