common.mk: sizes.c depends on PREP now

* common.mk (sizes.c): now depends on PREP, which is miniruby if
  native compile or fake.rb otherwise, to run MINIRUBY [Bug #8968]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43145 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-10-05

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43146 b2dd03c8-39d4-4d8f-98ff-823fe69b080e