vulkan-samples: Drop using u8string_view

Its deprecated in upstream fmt as well. Moreover it helps compile with
latest compiler

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>