allow bundled Vulkan lib also with newer cmakes
allow bundled Vulkan lib also with newer cmakes

Remove REQUIRED flag for the Vulkan library
search that are used if CMake version > 3.7.

This will allow the configuration and build fall back
of searching the internal vulkan library versions that
are bundled within this repo. (In case that the user
has not installed vulkan libraries under /usr)

Signed-off-by: Mika Laitio <7cbc313e10a1539892f48d7e18bf4c87b98df3b5@pilppa.org>

SaschaWillems/Vulkan,SaschaWillems/Vulkan,jpbruyere/Vulkan,grim210/Vulkan,SaschaWillems/Vulkan,SaschaWillems/Vulkan,ming4883/Vulkan,ming4883/Vulkan,grim210/Vulkan,ming4883/Vulkan,jpbruyere/Vulkan,SaschaWillems/Vulkan