Fixed warnings.

upb/msg.c

@@ -158,8 +158,8 @@ void upb_msg_addunknown(upb_msg *msg, const char *data, size_t len) {
   in->unknown_len += len;
 }
 
-const char *upb_msg_getunknown(upb_msg *msg, size_t *len) {
-  upb_msg_internal* in = upb_msg_getinternal(msg);
+const char *upb_msg_getunknown(const upb_msg *msg, size_t *len) {
+  const upb_msg_internal* in = upb_msg_getinternal_const(msg);
   *len = in->unknown_len;
   return in->unknown;
 }

upb/msg.h

@@ -155,7 +155,7 @@ upb_msg *upb_msg_new(const upb_msglayout *l, upb_arena *a);
 upb_arena *upb_msg_arena(const upb_msg *msg);
 
 void upb_msg_addunknown(upb_msg *msg, const char *data, size_t len);
-const char *upb_msg_getunknown(upb_msg *msg, size_t *len);
+const char *upb_msg_getunknown(const upb_msg *msg, size_t *len);
 
 /* Read-only message API.  Can be safely called by anyone. */