Make statically initialized char array vars_dir const
Make statically initialized char array vars_dir const

The array is meant to be const, it's just that g_strdupv takes gchar**
(non-const), likely just to make it easier to use with a non-const
argument. Change the type to const and cast it explicitly to shut down
the warning:

.../dnf-context.cpp:232:32: error: invalid conversion from 'const gchar**' {aka 'const char**'} to 'gchar**' {aka 'char**'} [-fpermissive]
     priv->vars_dir = g_strdupv((const gchar **) vars_dir);

Closes: #715
Approved by: Conan-Kudo

hughsie/libhif,rpm-software-management/libhif,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libdnf,rpm-software-management/libhif,rpm-software-management/libdnf,rpm-software-management/libdnf,rpm-software-management/libhif,hughsie/libhif