editorconfig: Set insert_final_newline true

This is what checkpatch.pl expects, and GitHub's file viewer flags a
missing trailing newline too.

Note that this is different from a blank line at the end of the file. It
just says whether the last line must end in a newline.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>