[media] v4l2-ctrls.h: fix doc reference for prepare_ext_ctrls()

The prepare_ext_ctrls() function is actually internal to the
v4l2-ctrls.c implementation, so it doesn't have a declaration
for the kAPI header to reference it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>