kern/52790: kernel compile failing with undefined reference to `ether_enable_vlan_mtu'

[scole_mail%gmx.com@localhost]

>Number:         52790
>Category:       kern
>Synopsis:       kernel compile failing with undefined reference to `ether_enable_vlan_mtu'
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Dec 07 18:00:00 +0000 2017
>Originator:     scole_mail
>Release:        current
>Organization:
none
>Environment:
NetBSD dstar 7.1.0_PATCH NetBSD 7.1.0_PATCH (GENERIC) #1: Wed Nov 29 14:17:46 PST 2017  scole@dstar:/home/scole/nbsd/cvs/7_0/obj/sys/arch/i386/compile/GENERIC i386
>Description:
There are a few kernels failing to compile with an error like

#      link  GENERIC.SKI/netbsd
/home/scole/nbsd/cvs/src/obj/tooldir.NetBSD-7.1.0_PATCH-i386/bin/ia64--netbsd-ld -Map netbsd.map --cref -T netbsd.ldscript -e start -X -o netbsd ${SYSTEM_OBJ:[@]:Nswapnetbsd.o} ${EXTRA_OBJ} vers.o swapnetbsd.o
if.o: In function `if_enable_vlan_mtu':
/home/scole/nbsd/cvs/src/sys/net/if.c:3623: undefined reference to `ether_enable_vlan_mtu'
if.o: In function `if_disable_vlan_mtu':
/home/scole/nbsd/cvs/src/sys/net/if.c:3635: undefined reference to `ether_disable_vlan_mtu'
*** [netbsd] Error code 1

http://releng.netbsd.org/builds/HEAD/201712071010Z/atari.build.failed
http://releng.netbsd.org/builds/HEAD/201712071010Z/ia64.build.failed

It seems like ether_enable_vlan_mtu() is not being included from if_ethersubr.c unless enough network options are added in the kernel config.

These kernels used to compile not long ago.
>How-To-Repeat:
I think having a kernel without any network options but maybe these

options 	INET		# IP + ICMP + TCP + UDP
pseudo-device	loop		# network loopback


>Fix: