Add some missing returns pointed out by -Wreturn-type:
-----------------------------------------------------------------------
[32/315] Building C object src/xrt/auxiliary/CMakeFiles/aux_gstreamer.dir/gstreamer/gst_sink.c.o
.../src/xrt/auxiliary/gstreamer/gst_sink.c:53:1: warning: non-void function does not return a value in all control paths [-Wreturn-type]
}
^
1 warning generated.
[84/315] Building C object src/xrt/auxiliary/CMakeFiles/aux_vk.dir/vk/vk_compositor_flags.c.o
.../src/xrt/auxiliary/vk/vk_compositor_flags.c:117:1: warning: non-void function does not return a value in all control paths [-Wreturn-type]
}
^
.../src/xrt/auxiliary/vk/vk_compositor_flags.c:146:1: warning: non-void function does not return a value in all control paths [-Wreturn-type]
}
^
2 warnings generated.
-----------------------------------------------------------------------
For the function returning VkImageAspectFlags return a literal 0 because
the enum values VK_IMAGE_ASPECT_NONE or VK_IMAGE_ASPECT_NONE_KHR may not
always be defined.
