BUG/MINOR: comp: fix compilation warning compiling without compression.

This is specific to threads, no backport is needed.
diff --git a/src/compression.c b/src/compression.c
index d476da5..594399a 100644
--- a/src/compression.c
+++ b/src/compression.c
@@ -41,9 +41,11 @@
 #include <proto/stream.h>
 
 
+#if defined(USE_SLZ) || defined(USE_ZLIB)
 #ifdef USE_THREAD
 static HA_SPINLOCK_T comp_pool_lock;
 #endif
+#endif
 
 #ifdef USE_ZLIB