Don't export `mrb_str_buf_cat`

Don't export `mrb_str_buf_cat` which was inlined in mruby/mruby@bc23a5e9e555f7a0b606856c417b9910ed92af68.
Merge pull request #6 from SHyx0rmZ/remove-str-buf-cat

Don't export `mrb_str_buf_cat`