kernel: add @IPV6 dependency to ipv6 modules

kernel: add @IPV6 dependency to ipv6 modules

IPv6 modules should all depend on @IPV6, to avoid circular dependencies
problems, especially if they select a module that depends on IPV6 as
well.  In theory, if a package A depends on IPV6, any package doing
'select A' (DEPENDS+= A) should also depend on IPV6; otherwise selecting
A will fail.  Sometimes the build system is forgiving this, but
eventually, and unexpectedly, it may blow up on some other commit.

Alternatively one can conditionally add IPv6 dependencies only if
CONFIG_IPV6 is selected: (DEPENDS+= +IPV6:package6).

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>