Add access to gp_library_version function

[h3x4d3c1m4l]

Usage example:

use ::gphoto2::libgphoto2_sys::{gp_library_version, GPVersionVerbosity};

pub fn get_version_info() -> String {
    // Returns "2.5.31" as of now if latest stable is available
    unsafe {
        CStr::from_ptr(*gp_library_version(GPVersionVerbosity::GP_VERSION_SHORT))
    }.to_string_lossy().into_owned()
}

[maxicarlos08]

@h3x4d3c1m4l Published a new version and also included a utility function in the gphoto2 crate similar to yours

[h3x4d3c1m4l]

Awesome, thanks for merging!