Security update for sddm (moderate)

This update for sddm fixes the following issues:

The following security vulnerability was addressed:

• CVE-2018-14345: Fixed the authentication, which did not check the
  password for users with an already existing session and allowed any user
  with access to the system bus to unlock any graphical session.
  (boo#1101450)

The following other bugs were addressed:

• Fallback to embedded theme, if none is set
• Corrected section name for Wayland
• Removed patch, which is no longer needed, because bug in libxcb was
  fixed in the meanwhile (boo#1099908)