BUILD: pools: import plock.h to build even without thread support

In 2.9-dev4, commit 544c2f2d9 ("MINOR: pools: use EBO to wait for
unlock during pool_flush()") broke the thread-less build by calling
pl_wait_new_long() without explicitly including plock.h which is
normally included by thread.h when threads are enabled.

src/pool.c

@@ -12,6 +12,8 @@
 
 #include <errno.h>
 
+#include <import/plock.h>
+
 #include <haproxy/activity.h>
 #include <haproxy/api.h>
 #include <haproxy/applet-t.h>