qemu: needs IPv6

Fixes http://autobuild.buildroot.net/results/f53/f53d4d47a6b65c40c1b122dfad61d9164e533bf0/



Actually only the linux-user code needs it, but with the _CUSTOM_TARGETS
option it is difficult to ensure it only for userland emulation, so just add
it to the main qemu option.

At the same time add the missing wchar/mmu dependencies to the comment.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>