cmake HAVE_STRUCT_IN6_ADDR examples

CTSRD-CHERI/cheribsd contrib/tcpdump/CMakeLists.txt :737 

check_type_size("struct in6_addr" HAVE_STRUCT_IN6_ADDR)

CTSRD-CHERI/cheribsd contrib/tcpdump/CMakeLists.txt :741 

endif(HAVE_AF_INET6 AND HAVE_STRUCT_IN6_ADDR)