configure: list libv4l2 as an optional library for v4l2

Fixes regression since 6dfcbd80
and 734ed389.
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -3116,7 +3116,6 @@ kmsgrab_indev_deps="libdrm"
 lavfi_indev_deps="avfilter"
 libcdio_indev_deps="libcdio"
 libdc1394_indev_deps="libdc1394"
-v4l2_indev_deps="libv4l2"
 openal_indev_deps="openal"
 opengl_outdev_deps="opengl"
 oss_indev_deps_any="soundcard_h sys_soundcard_h"
@@ -3127,7 +3126,9 @@ sdl2_outdev_deps="sdl2"
 sndio_indev_deps="sndio"
 sndio_outdev_deps="sndio"
 v4l2_indev_deps_any="linux_videodev2_h sys_videoio_h"
+v4l2_indev_suggest="libv4l2"
 v4l2_outdev_deps_any="linux_videodev2_h sys_videoio_h"
+v4l2_outdev_suggest="libv4l2"
 vfwcap_indev_deps="vfw32 vfwcap_defines"
 xcbgrab_indev_deps="libxcb"
 xcbgrab_indev_suggest="libxcb_shm libxcb_shape libxcb_xfixes"