Examples for functions under #ifdef CCAN_XXX_DEBUG should be compiled with it.

[rustyrussell]

For example, add the following to the description of ccan/timers/timers.h timers_dump:

*
* Example:
*   timers_dump(&timeouts, stdout);

Now ccanlint can't compile it, because timers_dump is only available with CCAN_TIMERS_DEBUG.