Specify memchr depenency via Git instead of version

This is a temporary fix for https://gitlab.com/ra_kete/kmod-rs to
circumvent Cargo issue rust-lang/cargo#2589.

By specifying the memchr dependency using its Git URL rather than its
crates.io version, it looks to Cargo different than the memchr
dependency specified by bindgen. Hence the features of both dependencies
are no longer unionized and we get a memchr free of libc.

The plan is to use this workaround only as long as the above Cargo issue
is not fixed. Afterwards we can just use the cstr_core release from
crates.io again.

Cargo.toml

@@ -11,7 +11,7 @@ keywords = ["cstr", "cstring", "no_std", "string", "c"]
 
 [dependencies]
 cty = "0.1"
-memchr = { version = "2.0", default-features = false }
+memchr = { git = "https://github.com/BurntSushi/rust-memchr.git", default-features = false }
 
 [features]
 alloc = []