compat: Load subr-x for Emacs < 28 compatibility
* lisp/org-compat.el: Require subr-x at compile time to define
string-trim for older Emacs versions.
Before Emacs 28.1, string-trim was defined in subr-x, leading to 'make
single' complaining about an undefined string-trim in the recently
added org-string-clean-whitespace.
Reported-by: Ihor Radchenko <yantar92@gmail.com>
Link: https://list.orgmode.org/8735c01aho.fsf@localhost