spec: use %autosetup

Replace the two %setup calls with %autosetup; the second call was even
redoing the first.

Reported by Neal Gompa, thanks!

packaging/cockpit-starter-kit.spec.in

@@ -25,8 +25,7 @@ Requires: cockpit-system
 Cockpit Starter Kit Example Module
 
 %prep
-%setup -q -n %{name}
-%setup -q -a 1 -n %{name}
+%autosetup -n %{name} -a 1
 # ignore pre-built webpack in release tarball and rebuild it
 rm -rf dist