Erroneous -Werror=missing-braces on old GCC

I came across the 'missing braces' warning again when building master
(0a93f803f4) on old GCC (4.8.5).

blkreftable.c: In function ‘BlockRefTableSetLimitBlock’:
blkreftable.c:268:2: warning: missing braces around initializer [-Wmissing-braces]
  BlockRefTableKey key = {0}; /* make sure any padding is zero */
  ^

This has popped up a few times in the past, and it seems to be GCC bug
53119.  We previously used the {{...}} approach to suppress it.  Should
we do the same here, like attached?

FWIW, in the same file we initialize BlockRefTableSerializedEntry
variables also with {{0}}.

Thanks
Richard