ffmpeg: fix build failure when vulkan is enabled

The patch fixes the following errors observed when building ffmpeg in
vulkan-enabled distros:

| src/libavutil/hwcontext_vulkan.c:363:7: error: 'VK_EXT_VIDEO_DECODE_H264_EXTENSION_NAME' undeclared here (not in a function); did you mean 'VK_EXT_VIDEO_ENCODE_H264_EXTENSION_NAME'?
|   363 |     { VK_EXT_VIDEO_DECODE_H264_EXTENSION_NAME,                FF_VK_EXT_NO_FLAG                },
|       |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|       |       VK_EXT_VIDEO_ENCODE_H264_EXTENSION_NAME
| src/libavutil/hwcontext_vulkan.c:364:7: error: 'VK_EXT_VIDEO_DECODE_H265_EXTENSION_NAME' undeclared here (not in a function); did you mean 'VK_EXT_VIDEO_ENCODE_H265_EXTENSION_NAME'?
|   364 |     { VK_EXT_VIDEO_DECODE_H265_EXTENSION_NAME,                FF_VK_EXT_NO_FLAG                },
|       |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|       |       VK_EXT_VIDEO_ENCODE_H265_EXTENSION_NAME

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>