org-src: Silence byte-compiler under 'make single'

This warning shows up when compiling in the Emacs repo, too.

lisp/org-src.el

@@ -38,6 +38,7 @@
 (require 'org-keys)
 
 (declare-function org-mode "org" ())
+(declare-function org--get-expected-indentation "org" (element contentsp))
 (declare-function org-element-at-point "org-element" ())
 (declare-function org-element-class "org-element" (datum &optional parent))
 (declare-function org-element-context "org-element" (&optional element))