clang does not like the array index assignment without the `=` sign in
it. This is a gnu extension I believe, and adding the sign is proper.
This fixes the following two warnings:
```
../subprojects/gst-plugins-bad/gst-libs/gst/vulkan/gstvkvideo-private.c:32:40:
warning: use of GNU 'missing =' extension in designator [-Wgnu-designator]
[GST_VK_VIDEO_EXTENSION_DECODE_H264] {
^
=
../subprojects/gst-plugins-bad/gst-libs/gst/vulkan/gstvkvideo-private.c:36:40:
warning: use of GNU 'missing =' extension in designator [-Wgnu-designator]
[GST_VK_VIDEO_EXTENSION_DECODE_H265] {
^
=
```
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5996>
