Add 'BuildRequires: golang(github.com/spf13/viper)'

Earlier Viper was being pulled in by Cobra, and hence wasn't explicitly
listed as a BuildRequires.  However, Cobra 1.4.0 removed the Viper
dependency [1], so it needs to be explicitly listed.

There's no need to do a build just for this.

[1] Cobra commit 5b2b9e9f61d36ccb
    spf13/cobra#1597

toolbox.spec

@@ -32,6 +32,7 @@ BuildRequires: golang(github.com/mattn/go-isatty) >= 0.0.12
 BuildRequires: golang(github.com/sirupsen/logrus) >= 1.4.2
 # BuildRequires: golang(github.com/stretchr/testify) >= 1.7.0
 BuildRequires: golang(github.com/spf13/cobra) >= 0.0.5
+BuildRequires: golang(github.com/spf13/viper) >= 1.3.2
 BuildRequires: golang(golang.org/x/sys/unix)
 BuildRequires: meson >= 0.58.0
 BuildRequires: pkgconfig(bash-completion)