fake.rb.in: fix make install failure

* template/fake.rb.in: fix make install failure due to MSYS path
  with mingw on MSYS environment.
  [ruby-core:64965] [Bug #10230]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47774 b2dd03c8-39d4-4d8f-98ff-823fe69b080e