rbconfig/sizeof: VPATH prefix

* ext/rbconfig/sizeof/depend (sizes.c): prefix with nmake-style
  VPATH to use the distributed source in out-of-place builds.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46924 b2dd03c8-39d4-4d8f-98ff-823fe69b080e