cmake-mode.el: Re-add explicit call to require thingatpt

Usually it is not needed to call '(require 'thingatpt')' explicitly
because the function 'symbol-at-point' is in autoloaded but to be sure
to have the function loaded in every case, require thingatpt.

Auxiliary/cmake-mode.el

@@ -320,6 +320,7 @@ and store the result as a list in LISTVAR."
       ))
   )
 
+(require 'thingatpt)
 (defun cmake-symbol-at-point ()
   (let ((symbol (symbol-at-point)))
     (and (not (null symbol))