dracut: Fix missing _arch variable declaration in 40network modules-setup.sh

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

Index: dracut-037/modules.d/40network/module-setup.sh
===================================================================
--- dracut-037.orig/modules.d/40network/module-setup.sh
+++ dracut-037/modules.d/40network/module-setup.sh
@@ -22,6 +22,7 @@ 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'