First, sc was undefined, then I tried myself and
applied this patch:

--- if_xennet.c.orig    2005-03-18 12:18:28.000000000 +0200
+++ if_xennet.c 2005-03-18 12:19:02.000000000 +0200
@@ -1105,6 +1105,7 @@
  #ifdef mediacode
         struct ifreq *ifr = (struct ifreq *)data;
  #endif
+       struct xennet_softc *sc = ifp->if_softc;
         int s, error = 0;

         s = splnet();

after which I got this, and gave up:

#   compile  INSTALL_XENU/if_xennet.o
/usr/obj/tools/bin/i386--netbsdelf-gcc -pipe -march=pentium4 -ffreestanding 
-Os -pipe -march=pentium4 -Werror -Wall -Wno-main -Wno-format-zero-length 
-Wpointer-arith -Wmissing-prototypes -Wstrict-prototypes -Wno-sign-compare 
-fno-zero-initialized-in-bss -Di386 -I. 
-I/usr/obj/usr/src/sys/arch/i386/compile/INSTALL_XENU/xen-ma 
-I/usr/src/sys/arch -I/usr/src/sys -nostdinc -DMATH_EMULATE -DMALLOC_NOINLINE 
-DMAXUSERS=32 -D_KERNEL -D_KERNEL_OPT -c /usr/src/sys/arch/xen/xen/if_xennet.c
/usr/src/sys/arch/xen/xen/if_xennet.c: In function `xennet_ioctl':
/usr/src/sys/arch/xen/xen/if_xennet.c:1108: warning: unused variable `sc'

BR,
Teemu

-- 
teemu@rinta-aho.org -+- http://www.rinta-aho.org