dir-locals: apply our C settings in C++ also

This also works with Emacs 30 Tree-Sitter C and C++ modes, as they are
submodes.

ChangeLog:

	* .dir-locals.el: Change c-mode to a list of C, C++ and ObjC
	modes that Emacs currently provides.

.dir-locals.el

@@ -18,6 +18,10 @@
 	      (tcl-continued-indent-level . 4)
 	      (indent-tabs-mode . t)))
  (nil . ((bug-reference-url-format . "https://gcc.gnu.org/PR%s")))
+ ;; Please keep C and C++ in sync.
  (c-mode . ((c-file-style . "GNU")
 	    (indent-tabs-mode . t)
-	    (fill-column . 79))))
+	    (fill-column . 79)))
+ (c++-mode . ((c-file-style . "GNU")
+	      (indent-tabs-mode . t)
+	      (fill-column . 79))))