MINOR: buffers: cache-align buffer_wq_lock

This lock is highly stressed, avoid cache-line sharing to limit stress.
diff --git a/src/buffer.c b/src/buffer.c
index 4e726b0..167b75a 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -33,7 +33,7 @@
 
 /* list of objects waiting for at least one buffer */
 struct list buffer_wq = LIST_HEAD_INIT(buffer_wq);
-__decl_hathreads(HA_SPINLOCK_T buffer_wq_lock);
+__decl_hathreads(HA_SPINLOCK_T __attribute__((aligned(64))) buffer_wq_lock);
 
 /* this buffer is always the same size as standard buffers and is used for
  * swapping data inside a buffer.