mruby.h: 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.
add missing declaration of mrb_str_format
Merge pull request #1670 from cremno/remove-mrb_str_format-declaration

remove mrb_str_format declaration