bug#26658: 25.2; cc-mode doesn't use prog-mode-map as parent for c-mode-base-map

[Vasilij Schneidermann]

> So yes, this change would surely be a good idea, given that this CC Mode
> is derived from prog-mode anyway.

The more I think about it, the less sure I become.  Pretty much every
CC-derived mode does (define-derived-mode my-mode prog-mode ...) anyway
and according to the macro expansion, its keymap's parent is set to
(current-local-map) which should be equivalent to `prog-mode-map'.
Could very well be a non-issue in practice and merely faulty testing by
the user...

> Do you want to write the patch?  If so, have you got copyright
> assignments, or can you manage to write it as an "insignificant" (i.e.
> very small) change?  I think the limit for insignificant changes is less
> than 15 lines, but I'm not absolutely sure.  This wouldn't even need any
> changes in the manual.  :-)
> 
> If you don't really want to write the patch, just say so, and I'll do
> it.

I've assigned copyright already, but reckon this is a one-line patch.
Before I submit anything, I'd like to make sure it's needed and the
change works out as expected.