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

lisp/org-compat.el

@@ -34,6 +34,8 @@
 (require 'seq)
 (require 'org-macs)
 
+(eval-when-compile (require 'subr-x))  ; Emacs < 28
+
 ;; We rely on org-compat when generating Org version.  Checking Org
 ;; version here will interfere with Org build process.
 ;; (org-assert-version)