Fix license attributions for src/3rdparty/glslang

Following files do have a copyright that is not one of the 2 standard ones:

Khronos License

• qtshadertools/src/3rdparty/glslang/SPIRV/spirv.hpp
• qtshadertools/src/3rdparty/glslang/SPIRV/GLSL.std.450.h
• qtshadertools/src/3rdparty/glslang/SPIRV/GLSL.ext.NV.h
• qtshadertools/src/3rdparty/glslang/SPIRV/GLSL.ext.KHR.h
• qtshadertools/src/3rdparty/glslang/SPIRV/GLSL.ext.EXT.h
• qtshadertools/src/3rdparty/glslang/SPIRV/GLSL.ext.AMD.h
• qtshadertools/src/3rdparty/glslang/glslang/MachineIndependent/gl_types.h

Apache 2.0 License

• qtshadertools/src/3rdparty/glslang/SPIRV/hex_float.h
• qtshadertools/src/3rdparty/glslang/SPIRV/bitutils.h

GNU General Public License v3.0 or later with Bison Exception 2.2

• qtshadertools/src/3rdparty/glslang/glslang/MachineIndependent/glslang_tab.cpp.h
• qtshadertools/src/3rdparty/glslang/glslang/MachineIndependent/glslang_tab.cpp

Anyhow, qtshadertools/src/3rdparty/glslang/LICENSE.txt does imply that some (all?) of them are not needed / used:

Bison was removed long ago. You can build glslang from the source grammar,
using tools of your choice, without using bison or any bison files.

Other parts, outside of glslang proper, include:

• gl_types.h, only needed for OpenGL-like reflection, and can be left out of
  a parse and codegen project. See it for its license.

• update_glslang_sources.py, which is not part of the project proper and does
  not need to be used.

• the SPIR-V "remapper", which is optional, but has the same license as
  glslang proper

• Google tests and SPIR-V tools, and anything in the external subdirectory
  are external and optional; see them for their respective licenses.

Please remove the files, if necessary. Or document the additional licenses.