gnu: emacs-pdf-tools: Add missing input.

This is a followup to commit eccd0b57a1.

* gnu/packages/emacs.scm (emacs-pdf-tools)[propagated-inputs]: Add
'let-alist'.

gnu/packages/emacs.scm

@@ -1057,6 +1057,8 @@ single buffer.")
                      ("automake" ,automake)
                      ("pkg-config" ,pkg-config)
                      ("emacs" ,emacs-no-x)))
+    (propagated-inputs
+     `(("let-alist" ,let-alist)))
     (inputs `(("poppler" ,poppler)
               ("cairo" ,cairo)
               ("glib" ,glib)