BUG/MINOR: cpuset: fix compilation on platform without cpu affinity

BUG/MINOR: cpuset: fix compilation on platform without cpu affinity

The compilation is currently broken on platform without USE_CPU_AFFINITY
set. An error has been reported by the cygwin build of the CI.

This does not need to be backported.

In file included from include/haproxy/global-t.h:27,
                 from include/haproxy/global.h:26,
                 from include/haproxy/fd.h:33,
                 from src/ev_poll.c:22:
include/haproxy/cpuset-t.h:32:3: error: #error "No cpuset support implemented on this platform"
   32 | # error "No cpuset support implemented on this platform"
      |   ^~~~~
include/haproxy/cpuset-t.h:37:2: error: unknown type name ‘CPUSET_REPR’
   37 |  CPUSET_REPR cpuset;
      |  ^~~~~~~~~~~
make: *** [Makefile:944: src/ev_poll.o] Error 1
make: *** Waiting for unfinished jobs....
In file included from include/haproxy/global-t.h:27,
                 from include/haproxy/global.h:26,
                 from include/haproxy/fd.h:33,
                 from include/haproxy/connection.h:30,
                 from include/haproxy/ssl_sock.h:27,
                 from src/ssl_sample.c:30:
include/haproxy/cpuset-t.h:32:3: error: #error "No cpuset support implemented on this platform"
   32 | # error "No cpuset support implemented on this platform"
      |   ^~~~~
include/haproxy/cpuset-t.h:37:2: error: unknown type name ‘CPUSET_REPR’
   37 |  CPUSET_REPR cpuset;
      |  ^~~~~~~~~~~
make: *** [Makefile:944: src/ssl_sample.o] Error 1