ruby.h: get rid of C++ warnings

* include/ruby/ruby.h (PRIsVALUE): put a space after string
  literals not to be confused with C++11 string literal suffix.
  https://github.com/ruby/ruby/commit/a9f3eb7#commitcomment-9040169

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48892 b2dd03c8-39d4-4d8f-98ff-823fe69b080e