(bugfix): Explicitly load subr-x library (#425)

Ensure the `when-let` macro definitions is available during
compile-time.

org-roam-db.el

@@ -31,6 +31,7 @@
 ;;
 ;;; Code:
 ;;;; Library Requires
+(require 'subr-x)
 (require 'emacsql)
 (require 'emacsql-sqlite)
 (require 'org-roam-macs)