missing dir when c_rehash.rb in Ruby40-x64

[uk-sinc]

If one is adding a certificate with c_rehash.rb it fails with error: "No such file or directory @ dir_initialize - C:/Ruby40-x64/msys64/ucrt64/etc/ssl/certs (Errno::ENOENT)".
The script has to check, if this dir exists and has to create it if not.

[larskanis]

This is new since #461 . I think the directory C:/Ruby40-x64/msys64/ucrt64/etc/ssl/certs is missing when MSYS2 is already installed but not the MINGW packages. Seems a good idea to create the directory beforehand and put the certificate into, so that a later package install will find it.