conf: Fix clang-tidy warning about using an undefined enum value

In conf() we temporarily set the forwarding mode variables to 0 - an
invalid value, so that we can check later if they've been set by the
intervening logic.  clang-tidy 18.1.1 in Fedora 40 now complains about
this.  Satisfy it by giving an name in the enum to the 0 value.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

fwd.h

@@ -11,6 +11,7 @@
 #define	NUM_PORTS	(1U << 16)
 
 enum fwd_ports_mode {
+	FWD_UNSET = 0,
 	FWD_SPEC = 1,
 	FWD_NONE,
 	FWD_AUTO,