media: vidioc-g-ext-ctrls.rst: add missing 'struct' before the types

Add 'struct' to clarify that these are pointers to structs.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>