Add missing "C" ABI to FFI example code

[peeley]

In the "Calling foreign functions" section of the FFI chapter, an example extern block is given that reads like so:

#[link(name = "snappy")]
unsafe extern "C" {
    fn snappy_max_compressed_length(source_length: size_t) -> size_t;
}

This example is then expanded upon later in the section, with the following code:

#[link(name = "snappy")]
unsafe extern {
    fn snappy_compress(input: *const u8,
                       input_length: size_t,
                       compressed: *mut u8,
                       compressed_length: *mut size_t) -> c_int;
    fn snappy_uncompress(compressed: *const u8,
                         compressed_length: size_t,
                         uncompressed: *mut u8,
                         uncompressed_length: *mut size_t) -> c_int;
    fn snappy_max_compressed_length(source_length: size_t) -> size_t;
    fn snappy_uncompressed_length(compressed: *const u8,
                                  compressed_length: size_t,
                                  result: *mut size_t) -> c_int;
    fn snappy_validate_compressed_buffer(compressed: *const u8,
                                         compressed_length: size_t) -> c_int;
}

This expanded example is unfortunately missing the "C" ABI specification in the extern block declaration, which was present in the previous example. This PR adds the missing "C" ABI.

[ehuss]

Thanks!