C interface symbol export

[jayrulez]

Fixes this issue: https://github.com/KhronosGroup/glslang/issues/3001

An issue remains with the .lib for SPVRemapper not being generated when shared library build is enabled.

I don't know if anything should be exported there so I left as is.

[CLAassistant]

All committers have signed the CLA.

[greg-lunarg]

CI failures need to be addressed.

[arcady-lunarg]

Closing for inactivity. If you feel strongly that this should land, feel free to reopen or make a new PR.