Specify rust-version = "1.19" in Cargo.toml

The `libc-0.2` branch currently tests down to version 1.19. Document
this in Cargo.toml to give an obvious point of reference when this
changes.

Author: tgross35

Cargo.toml

@@ -11,6 +11,7 @@ keywords = ["libc", "ffi", "bindings", "operating", "system"]
 categories = ["external-ffi-bindings", "no-std", "os"]
 build = "build.rs"
 exclude = ["/ci/*", "/.github/*", "/.cirrus.yml", "/triagebot.toml"]
+rust-version = "1.19"
 description = """
 Raw FFI bindings to platform libraries like libc.
 """