Fix install path for bash-completion

CMAKE_INSTALL_DATADIR already includes the subdirectory share, so drop
it from the install path of the dunecontrol bash-completion script in
the corresponding CMakeLists.txt.

share/bash-completion/completions/CMakeLists.txt

 install(FILES
   dunecontrol
-  DESTINATION ${CMAKE_INSTALL_DATADIR}/share/bash-completion/
+  DESTINATION ${CMAKE_INSTALL_DATADIR}/bash-completion/completions/
 )