docs-rst: kernel-doc: fix typedef output in RST format

When using a typedef function like this one:
	typedef bool v4l2_check_dv_timings_fnc (const struct v4l2_dv_timings * t, void * handle);

The Sphinx C domain expects it to create a c:type: reference,
as that's the way it creates the type references when parsing
a c:function:: declaration.

So, a declaration like:

	.. c:function:: bool v4l2_valid_dv_timings (const struct v4l2_dv_timings * t, const struct v4l2_dv_timings_cap * cap, v4l2_check_dv_timings_fnc fnc, void * fnc_handle)

Will create a cross reference for :c:type:`v4l2_check_dv_timings_fnc`.

So, when outputting such typedefs in RST format, we need to handle
this special case, as otherwise it will produce those warnings:

	./include/media/v4l2-dv-timings.h:43: WARNING: c:type reference target not found: v4l2_check_dv_timings_fnc
	./include/media/v4l2-dv-timings.h:60: WARNING: c:type reference target not found: v4l2_check_dv_timings_fnc
	./include/media/v4l2-dv-timings.h:81: WARNING: c:type reference target not found: v4l2_check_dv_timings_fnc

So, change the kernel-doc script to produce a RST output for the
above typedef as:
	.. c:type:: v4l2_check_dv_timings_fnc

	   **Typedef**: timings check callback

	**Syntax**

	  ``bool v4l2_check_dv_timings_fnc (const struct v4l2_dv_timings * t, void * handle);``

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>