systemd: add missing dhcp.network file

This file was missing from commit
84d997d6890f1d045c4d6084148fd9b2d674a6c7 ("system(d): allow auto net
configuration with networkd")

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/systemd/dhcp.network

@@ -0,0 +1,4 @@
+[Match]
+Name=SYSTEMD_NETWORKD_DHCP_IFACE
+[Network]
+DHCP=ipv4