Newest GCC warns that the string copying is potentially truncated and
thus not nul terminated.

In file included from /usr/include/string.h:494,
                 from ../../src/Virt_HostSystem.c:23:
In function ‘strncpy’,
    inlined from ‘resolve_host’ at ../../src/Virt_HostSystem.c:55:28,
    inlined from ‘get_fqdn’ at ../../src/Virt_HostSystem.c:92:23,
    inlined from ‘set_host_system_properties’ at ../../src/Virt_HostSystem.c:109:13:
/usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 256 equals destination size [-Werror=stringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘strncpy’,
    inlined from ‘resolve_host’ at ../../src/Virt_HostSystem.c:67:17,
    inlined from ‘get_fqdn’ at ../../src/Virt_HostSystem.c:92:23,
    inlined from ‘set_host_system_properties’ at ../../src/Virt_HostSystem.c:109:13:
/usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound 256 equals destination size [-Werror=stringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>