uml: fix an IPV6 libc vs kernel symbol clash

On some systems, with IPV6 configured, there is a clash between the kernel's
in6addr_any and the one in libc.

This is handled in the usual (gross) way of defining the kernel symbol out of
the way on the gcc command line.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>