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)

Cargo.toml

@@ -30,7 +30,6 @@ limits = []
 time = "0.1.0"
 bitflags = "0.7"
 lru-cache = "0.1.0"
-libc = "0.2"
 chrono = { version = "0.3", optional = true }
 serde_json = { version = "0.9", optional = true }