Remove the dependency on libc

Recent versions of bindgen use `std::os::raw` over `libc`, but currently
`libsqlite3-sys` is overriding that. `std::os::raw` is a subset of
`libc` that exports only the relevant type definitions, but not any
functions which require additional linking. This enables
`libsqlite3-sys` to be more easily used on targets that may not have a
libc available (presumably sqlite itself would have been compiled with
musl in that case)

libsqlite3-sys/src/error.rs

@@ -1,4 +1,4 @@
-use libc::c_int;
+use std::os::raw::c_int;
 use std::error;
 use std::fmt;
 

libsqlite3-sys/src/lib.rs

@@ -1,7 +1,5 @@
 #![allow(non_snake_case, non_camel_case_types)]
 
-extern crate libc;
-
 pub use self::error::*;
 
 use std::mem;