common.mk: revert r43137 and r43145

* common.mk (sizes.c): revert r43137 and r43145, because the former
  commit made sizes.c rebuild always each build.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43147 b2dd03c8-39d4-4d8f-98ff-823fe69b080e