package/vde2: fix build with BR2_OPTIMIZE_0

Fix the following build failure with BR2_OPTIMIZE_0:

/home/autobuild/autobuild/instance-11/output-1/host/lib/gcc/mips64-buildroot-linux-uclibc/10.3.0/../../../../mips64-buildroot-linux-uclibc/bin/ld: vde_l3.o: in function `ip_output_ready':
vde_l3.c:(.text+0xb30): undefined reference to `iphead'

Fixes:
 - http://autobuild.buildroot.org/results/5eeefcdac0b8974a424a885fdc2e82f7cb4b617d



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>