remove mrb_str_format declaration

The function ```mrb_str_format``` is only defined when the mrbgem __mruby-sprintf__ was
activated. That might not always be the case (by default it currently is).
```mrb_str_format``` still has external linkage, so from now on mrbgem authors
have to declare it in their source code if they want or need to call it.

[mruby-syslog](https://github.com/iij/mruby-syslog) (cc: @tsahara-iij) and [mruby-WiringPi](https://github.com/akiray03/mruby-WiringPi) (cc: @akiray03) are affected by this.