databases/memcached: fix build on powerpc64le
In file included from hash.c:7:
In file included from ./xxhash.h:2934:
/usr/lib/clang/11.0.1/include/altivec.h:55:19: error: unknown type name 'vector'
static __inline__ vector bool char __ATTRS_o_ai
^
/usr/lib/clang/11.0.1/include/altivec.h:56:10: error: unknown type name 'vector'
vec_perm(vector bool char __a, vector bool char __b, vector unsigned char __c);