vim: Revert "no tab in CMake-files"

At some point setlocal et was added to indent/cmake.vim without much of an explanation. The commit message was "no tab in CMake-files".

Personally, I always use tabs to indent cmake files. To my knowledge there is no reason why that would cause problems. Using tabs or spaces is a personal choice (and the subject of many holy wars), so I would prefer if cmake does not try to enforce anything here.

Instead, I feel cmake should respect whatever choice the user made in their own vim configuration.

Topic-rename: revert-vim-no-tabs