package/memcached: disable -Werror

Disable -Werror to avoid the following build failure:

In file included from hash.c:7:
xxhash.h:2667:5: error: #warning is a GCC extension [-Werror]
 2667 | #   warning "XXH3 is highly inefficient without ARM or Thumb-2."
      |     ^~~~~~~
xxhash.h:2667:5: error: #warning "XXH3 is highly inefficient without ARM or Thumb-2." [-Werror=cpp]

Fixes:
 - http://autobuild.buildroot.org/results/3124bae73c207f1a118e57e41e222ef464ccb297



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>