vulkan: silence gobject-instrospection warnings

According with gi annotations [1] the "optional" annotation must be only
for (out) or (inout) parameters.

1. https://gi.readthedocs.io/en/latest/annotations/giannotations.html

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4244>

subprojects/gst-plugins-bad/gst-libs/gst/vulkan/gstvkfence.c

@@ -87,7 +87,7 @@ gst_vulkan_fence_free (GstVulkanFence * fence)
 /**
  * gst_vulkan_fence_new:
  * @device: the parent #GstVulkanDevice
- * @error: (optional): a #GError for the failure condition
+ * @error: (out) (optional): a #GError for the failure condition
  *
  * Returns: a new #GstVulkanFence or %NULL on error
  *

subprojects/gst-plugins-bad/gst-libs/gst/vulkan/gstvkinstance.c

@@ -1095,7 +1095,7 @@ gst_vulkan_instance_get_proc_address (GstVulkanInstance * instance,
 /**
  * gst_vulkan_instance_create_device:
  * @instance: a #GstVulkanInstance
- * @error: (optional): a #GError
+ * @error: (out) (optional): a #GError
  *
  * Returns: (transfer full): a new #GstVulkanDevice
  *

subprojects/gst-plugins-bad/gst-libs/gst/vulkan/gstvkqueue.c

@@ -112,7 +112,7 @@ gst_vulkan_queue_get_device (GstVulkanQueue * queue)
 /**
  * gst_vulkan_queue_create_command_pool:
  * @queue: a #GstVulkanQueue
- * @error: (optional): a #GError
+ * @error: (out) (optional): a #GError
  *
  * Returns: (transfer full): a new #GstVulkanCommandPool or %NULL
  *

subprojects/gst-plugins-bad/gst-libs/gst/vulkan/gstvkutils.c

@@ -489,7 +489,7 @@ gst_vulkan_get_or_create_image_view (GstVulkanImageMemory * image)
  * @device: a #GstVulkanDevice
  * @code: the SPIR-V shader byte code
  * @size: length of @code.  Must be a multiple of 4
- * @error: (optional): a #GError to fill on failure
+ * @error: (out) (optional): a #GError to fill on failure
  *
  * Returns: (transfer full): a #GstVulkanHandle for @image matching the
  *                           original layout and format of @image or %NULL