Remove pointless const in prototype

src/serd_internal.h

@@ -382,7 +382,7 @@ is_windows_path(const uint8_t* path)
 
 size_t
 serd_substrlen(const uint8_t* str,
-               const size_t   len,
+               size_t         len,
                size_t*        n_bytes,
                SerdNodeFlags* flags);