in6_ifattach: include sys/eventhandler.h to unbreak NOVIMAGE builds

LINT-NOVIMAGE fails to build due to a missing eventhandler.h include
which in hte VIMAGE case is likely leaked through some other header.
Add the #include to unbreak the build.

Fixes: 0d469d23715d6 (net: attach IPv4 and IPv6 stacks to an ...)