message: add dependencies for `pkg-config`
  Otherwise, it complains:
  run pkg_config fail: "Failed to run `\"pkg-config\" \"--libs\" \"--cflags\" \"openssl\"`: No such file or directory (os error 2)"
diff: @@ -22,6 +22,7 @@ Cargo requires the following tools and packages to build:
  * `git`
  * `python`
  * `curl` (on Unix)
  +* `pkg-config` (on Unix, used to figure out the `libssl` headers/libraries)
  * OpenSSL headers (only for Unix, this is the `libssl-dev` package on ubuntu)
  * `cargo` and `rustc`
  