fix: wrong File name in workspaceoptions_plasmasettings.kcfgc

It happened to be an warning because *.kcfg[c] base filenames matches,
but it's an error per se:

CMake Warning at /home/bam/kde/usr/lib64/cmake/KF5Config/KF5ConfigMacros.cmake:52 (message):

  /home/bam/kde/src/plasma-desktop/kcms/workspaceoptions/workspaceoptions_plasmasettings.kcfgc:
  Broken "File" field, make sure it's pointing at the right file
Call Stack (most recent call first):
  kcms/workspaceoptions/CMakeLists.txt:19 (kconfig_add_kcfg_files)