Mention libc from crates.io in TRPL: FFI

Fixes #29762

Author: steveklabnik

src/doc/trpl/ffi.md

@@ -8,6 +8,23 @@ foreign code. Rust is currently unable to call directly into a C++ library, but
 snappy includes a C interface (documented in
 [`snappy-c.h`](https://github.com/google/snappy/blob/master/snappy-c.h)).
 
+## A note about libc
+
+Many of these examples use [the `libc` crate][libc], which provides various
+type definitions for C types, among other things. If you’re trying these
+examples yourself, you’ll need to add `libc` to your `Cargo.toml`:
+
+```toml
+[dependencies]
+libc = "0.2.0"
+```
+
+[libc]: https://crates.io/crates/libc
+
+and add `extern crate libc;` to your crate root.
+
+## Calling foreign functions
+
 The following is a minimal example of calling a foreign function which will
 compile if snappy is installed: