BUILD: cpuset: do not use const on the source of CPU_AND/CPU_ASSIGN

BUILD: cpuset: do not use const on the source of CPU_AND/CPU_ASSIGN

The man page indicates that CPU_AND() and CPU_ASSIGN() take a variable,
not a const on the source, even though it doesn't make much sense. But
with older libcs, this triggers a build warning:

  src/cpuset.c: In function 'ha_cpuset_and':
  src/cpuset.c:53: warning: initialization discards qualifiers from pointer target type
  src/cpuset.c: In function 'ha_cpuset_assign':
  src/cpuset.c:101: warning: initialization discards qualifiers from pointer target type

Better stick stricter to the documented API as this is really harmless
here. There's no need to backport it (unless build issues are reported,
which is quite unlikely).