tags: put function prototypes back!

Commit 7db86dc (ctags: usability fix) removed function prototypes from
tags file claiming "It makes no real sense to include function
prototypes".
But it is useful for quickly determining which header file developer
needs to include to fix compilation.

Now if someone wants to remove forward declarations (which I agree are
baggage), write a postprocessing script.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>