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/Cargo.toml

@@ -15,6 +15,3 @@ bundled = []
 bindgen = "0.21"
 pkg-config = "0.3"
 gcc = "0.3"
-
-[dependencies]
-libc = "0.2"