When hotdoc documentation is enabled and opencv plugin is set as
auto-detected, but the library isn't installed, meson configuration fails
with this message:
../subprojects/gst-plugins-bad/docs/meson.build:139:21: ERROR: Unknown variable "gstopencv_dep".
This patch fixes this case defined gstopencv_dep as disabler() when
dependencies aren't found.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5560>