build: work around broken kernel header

build: work around broken kernel header

I got this scary warning during ./configure on rawhide:

checking linux/if_bridge.h usability... no
checking linux/if_bridge.h presence... yes
configure: WARNING: linux/if_bridge.h: present but cannot be compiled
configure: WARNING: linux/if_bridge.h:     check for missing prerequisite headers?
configure: WARNING: linux/if_bridge.h: see the Autoconf documentation
configure: WARNING: linux/if_bridge.h:     section "Present But Cannot Be Compiled"
configure: WARNING: linux/if_bridge.h: proceeding with the compiler's result
configure: WARNING:     ## ------------------------------------- ##
configure: WARNING:     ## Report this to libvir-list@redhat.com ##
configure: WARNING:     ## ------------------------------------- ##
checking for linux/if_bridge.h... no

* configure.ac (AC_CHECK_HEADERS): Provide struct in6_addr, since
linux/if_bridge.h uses it without declaring it.