Fails to build if using shared-only >=glslang-11.12.0

>=glslang-11.12.0 no longer installs any static libs if -DBUILD_SHARED_LIBS=ON (https://github.com/KhronosGroup/glslang/commit/7cd519511) and this attempts to use some static-only libs:

Dependency shaderc skipped: feature shaderc disabled
Library glslang found: YES
Library MachineIndependent found: NO

../libplacebo-v5.229.2/src/glsl/meson.build:19:2: ERROR: C++ shared or static library 'OSDependent' not found

Unsure if right fix, but builds fine against both static and shared if mark OSDependent and OGLCompiler as required: false.

(this is on Gentoo, builds with shaderc by default but users can pick glslang if they want -- Edit: although won't be able to reproduce on Gentoo given it's being worked around for now)