meson: fix opencv=disabled case if opencv is available on the system

ext/opencv/meson.build:103:2: ERROR:  Unknown variable "gstopencv_dep".
6 jobs for meson-fix-opencv-disabled-but-available in 69 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #65879
gst indent

00:00:41

passed #65878
manifest

00:00:25

 
  Build
passed #65880
build fedora x86_64

00:03:12

 
  Test
passed #65881
check fedora

00:15:56

 
  Full Builds
passed #65882
build android arm64 api28

00:02:31

passed #65883
build cerbero fedora x86_64

00:46:39