Add extern "C" to all header files

[bekuaghub]

I tried to integrate this project to a larger cpp project, there were errors when linking functions to the main project. There are some header files within the project, which already contain extern "C" definition, but most of them are missing that. It would be convenient to add extern "C" definitions to all header files to prevent future complications. Here is what to add:

#ifdef __cplusplus
extern "C" {
#endif

// mp function declarations

#ifdef __cplusplus
}
#endif