umdns: suppress address-of-packed-member warning

umdns: suppress address-of-packed-member warning

gcc 8 & 9 appear to be more picky with regards access alignment to
packed structures, leading to this warning in dns.c:

dns.c:261:2: error: converting a packed ‘struct dns_question’ pointer
(alignment 1) to a ‘uint16_t’ {aka ‘short unsigned int’} pointer
(alignment 2) may result in an unaligned pointer value
[-Werror=address-of-packed-member]

261 |  uint16_t *swap = (uint16_t *) q;

Work around what I think is a false positive by turning the warning off.
Not ideal, but not quite as not ideal as build failure.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
(cherry picked from commit 02640f014719a994e2e538b2cb6376a189cd39de)