There's no need for the low-level pool functions to be known from all
callers anymore, they're only used by pool.c. Let's reduce the amount
of header files processed.
#include <haproxy/api.h>
#include <haproxy/freq_ctr.h>
#include <haproxy/list.h>
-#include <haproxy/pool-os.h>
#include <haproxy/pool-t.h>
#include <haproxy/thread.h>
#include <haproxy/global.h>
#include <haproxy/list.h>
#include <haproxy/pool.h>
+#include <haproxy/pool-os.h>
#include <haproxy/sc_strm.h>
#include <haproxy/stats-t.h>
#include <haproxy/stconn.h>