spec: Add BR on toolset rust and runtime

The `cargo` BR was already pulling in the `rust` one, though to be clear
and look closer to what the guidelines suggest, let's make that
explicit.

Also guidelines suggest we should add a BR on the `-runtime` package as
well. It includes RPM macros to help with toolset stuff (although we're
not making use of them right now).

rpm-ostree.spec

@@ -30,6 +30,8 @@ URL: https://github.com/projectatomic/rpm-ostree
 ExclusiveArch: %{rust_arches}
     %if %{defined rusttoolset_version}
 BuildRequires: %{rusttoolset_version}-cargo
+BuildRequires: %{rusttoolset_version}-rust
+BuildRequires: %{rusttoolset_version}-runtime
     %else
         # This one is only in Fedora, we're not actually using it right now
         # but we may in the future.