BUILD: fix recent build breakage of freebsd caused by kFreeBSD build fix

BUILD: fix recent build breakage of freebsd caused by kFreeBSD build fix

Supporting kFreebsd previously led to FreeBSD (< 14) build breakage:

 In file included from src/cpuset.c:5:
 In file included from include/haproxy/cpuset.h:4:
 include/haproxy/cpuset-t.h:46:2: error: unknown type name 'cpu_set_t'; did you mean 'cpuset_t'?
         CPUSET_REPR cpuset;
         ^~~~~~~~~~~
         cpuset_t
 include/haproxy/cpuset-t.h:21:22: note: expanded from macro 'CPUSET_REPR'
 # define CPUSET_REPR cpu_set_t
                      ^

(cherry picked from commit 6709538068752ead403648ac5836f610d2da79e8)
Signed-off-by: Willy Tarreau <w@1wt.eu>