net: if: Make gateway address in net_if_ipv4_set_gw() const

As the function does not modify the parameter, we can make it const.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>

include/net/net_if.h

diff --git a/include/net/net_if.h b/include/net/net_if.h
index 19e0d44..a4f22b8 100644
--- a/include/net/net_if.h
+++ b/include/net/net_if.h
@@ -1672,7 +1672,7 @@
  * @param iface Interface to use.
  * @param gw IPv4 address of an gateway
  */
-void net_if_ipv4_set_gw(struct net_if *iface, struct in_addr *gw);
+void net_if_ipv4_set_gw(struct net_if *iface, const struct in_addr *gw);
 
 /**
  * @brief Set IPv4 gateway for an interface index.