graphics/mesa-devel: unbreak radv=OFF VKLAYERS=off after f36f3675dc48

meson.build:626:15: ERROR: Program 'glslangValidator' not found or not executable

https://gitlab.freedesktop.org/mesa/mesa/-/commit/41a929d94ca5
https://gitlab.freedesktop.org/mesa/mesa/-/commit/829d74b2f2db

PR: 280071
Reported by: Anton Saietskii