bpo-39943: Add the const qualifier to pointers on non-mutable PyUnicode data. (GH-19345)

diff --git a/Modules/cjkcodecs/multibytecodec.h b/Modules/cjkcodecs/multibytecodec.h
index 4d2b355..5946821 100644
--- a/Modules/cjkcodecs/multibytecodec.h
+++ b/Modules/cjkcodecs/multibytecodec.h
@@ -30,7 +30,7 @@
 typedef int (*mbcodec_init)(const void *config);
 typedef Py_ssize_t (*mbencode_func)(MultibyteCodec_State *state,
                         const void *config,
-                        int kind, void *data,
+                        int kind, const void *data,
                         Py_ssize_t *inpos, Py_ssize_t inlen,
                         unsigned char **outbuf, Py_ssize_t outleft,
                         int flags);