40network: add missing _arch variable declaration

The module_setup.sh script was missing an '_arch' declaration,
causing network not to be installed on s390.

Signed-off-by: Thomas Renninger <trenn@suse.de>

modules.d/40network/module-setup.sh

@@ -17,6 +17,7 @@ depends() {
 # called by dracut
 installkernel() {
     # Include wired net drivers, excluding wireless
+    local _arch=$(uname -m)
 
     net_module_filter() {
         local _net_drivers='eth_type_trans|register_virtio_device|usbnet_open'