rbconfig/sizeof: revert r46924

* ext/rbconfig/sizeof/depend: revert r46924, VPATH prefix, to fix
  build error on old FreeBSD.  NMAKE must die.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46955 b2dd03c8-39d4-4d8f-98ff-823fe69b080e