graphics/glslang: track static library dependency after cbf133ac

$ cat CMakeLists.txt
cmake_minimum_required(VERSION 3.10)
project(foo)
find_package(glslang REQUIRED)

$ cmake .
[...]
CMake Error at /usr/local/share/cmake/Modules/CMakeFindDependencyMacro.cmake:76 (find_package):
  By not providing "FindSPIRV-Tools-opt.cmake" in CMAKE_MODULE_PATH this
  project has asked CMake to find a package configuration file provided by
  "SPIRV-Tools-opt", but CMake did not find one.

  Could not find a package configuration file provided by "SPIRV-Tools-opt"
  with any of the following names:

    SPIRV-Tools-optConfig.cmake
    spirv-tools-opt-config.cmake

  Add the installation prefix of "SPIRV-Tools-opt" to CMAKE_PREFIX_PATH or
  set "SPIRV-Tools-opt_DIR" to a directory containing one of the above files.
  If "SPIRV-Tools-opt" provides a separate development package or SDK, be
  sure it has been installed.
Call Stack (most recent call first):
  /usr/local/lib/cmake/glslang/glslang-config.cmake:28 (find_dependency)
  CMakeLists.txt:3 (find_package)

https://github.com/KhronosGroup/glslang/commit/82e0d00b32d2

PR:		283489
Reported by:	Stefan Schlosser