cmake-mode.el: Require the rx package

We're using rx in cmake-mode.el. Technically, we don't have to load the
package, because it's already loaded at startup. But it's good style to
explicitly require the packages we use, if only to give a quick overview
of the package dependencies.

Auxiliary/cmake-mode.el

@@ -28,6 +28,7 @@
 ;;
 
 (require 'rst)
+(require 'rx)
 
 (defcustom cmake-mode-cmake-executable "cmake"
   "*The name of the cmake executable.