Added extern "C" block to glsl_optimizer.h

[elizagamedev]

This makes it easier to use the library in C or another programming language like Rust which can interface with the C ABI.

[dankcushions]

i think you also need to do

#ifdef __cplusplus
struct glslopt_shader;
struct glslopt_ctx;
#else
typedef struct {} glslopt_shader;
typedef struct {} glslopt_ctx;
#endif