.dir-locals.el: Set indent-tabs-mode to nil in Elisp sources
Despite .dir-locals.el having a catchall value of t for
indent-tabs-mode, Org's Elisp files are a mix of tabs and spaces.
Emacs has used a value of nil for indent-tabs-mode since 93d4412046
(Set indent-tabs-mode to nil in (most) Elisp sources, 2015-04-15). Do
the same.
Ref: https://orgmode.org/list/87eejsg9vw.fsf@gnu.org