add vulkan USE flag

Author: fzchen-dev

make.conf

@@ -11,6 +11,7 @@ USE="${USE} mbo libglvnd"
 USE="${USE} -kernel-5_4 kernel-5_15 hevc_codec"
 USE="${USE} intel-common"
 USE="${USE} gpu_sandbox_start_early"
+USE="${USE} vulkan"
 # USE="${USE} houdini houdini64" archero will fail to run with houdini
 #PORTAGE_BINHOST is pulled in from prebuilt.conf
 VIDEO_CARDS="intel iHD iHD_g11 iHD_g12 iHD_g8 iHD_g9"