py/qstr: Use `const` consistently to avoid a cast.

Originally at adafruit#4707

Signed-off-by: Artyom Skrobov <tyomitch@gmail.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/py/qstr.h b/py/qstr.h
index 19672afc0..ded105760 100644
--- a/py/qstr.h
+++ b/py/qstr.h
@@ -64,7 +64,7 @@ typedef uint16_t qstr_len_t;
 #endif
 
 typedef struct _qstr_pool_t {
-    struct _qstr_pool_t *prev;
+    const struct _qstr_pool_t *prev;
     size_t total_prev_len;
     size_t alloc;
     size_t len;