emacs: Fix "not defined at runtime" warning

Previously, the Emacs byte compiler produced the warning

    the function `remove-if-not' might not be defined at runtime.

because we only required cl at compile-time (not runtime).  This fixes
this warning by requiring cl at runtime, ensuring that the definition
of remove-if-not is available.

emacs/notmuch-lib.el

@@ -24,7 +24,7 @@
 (require 'mm-view)
 (require 'mm-decode)
 (require 'json)
-(eval-when-compile (require 'cl))
+(require 'cl)
 
 (defvar notmuch-command "notmuch"
   "Command to run the notmuch binary.")