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
                      ^
diff --git a/include/haproxy/cpuset-t.h b/include/haproxy/cpuset-t.h
index 984df8d..b770b17 100644
--- a/include/haproxy/cpuset-t.h
+++ b/include/haproxy/cpuset-t.h
@@ -16,7 +16,8 @@
 
 #include <haproxy/api-t.h>
 
-#if defined(__linux__) || defined(__DragonFly__) || defined(__FreeBSD_kernel__)
+#if defined(__linux__) || defined(__DragonFly__) || \
+  (defined(__FreeBSD_kernel__) && defined(__GLIBC__))
 
 # define CPUSET_REPR cpu_set_t
 # define CPUSET_USE_CPUSET