BUILD: compression: remove a warning when no compression lib is used

This warning appears when building without any compression library :

src/compression.c:143:19: warning: unused function 'init_comp_ctx' [-Wunused-function]
static inline int init_comp_ctx(struct comp_ctx **comp_ctx)
                  ^
src/compression.c:176:19: warning: unused function 'deinit_comp_ctx' [-Wunused-function]
static inline int deinit_comp_ctx(struct comp_ctx **comp_ctx)

No backport is needed.
diff --git a/src/compression.c b/src/compression.c
index 4e13bae..02ff724 100644
--- a/src/compression.c
+++ b/src/compression.c
@@ -56,7 +56,6 @@
 #endif
 
 unsigned int compress_min_idle = 0;
-static struct pool_head *pool_comp_ctx = NULL;
 
 static int identity_init(struct comp_ctx **comp_ctx, int level);
 static int identity_add_data(struct comp_ctx *comp_ctx, const char *in_data, int in_len, struct buffer *out);
@@ -137,6 +136,8 @@
 	return -1;
 }
 
+#if defined(USE_ZLIB) || defined(USE_SLZ)
+static struct pool_head *pool_comp_ctx = NULL;
 /*
  * Alloc the comp_ctx
  */
@@ -186,6 +187,7 @@
 #endif
 	return 0;
 }
+#endif
 
 
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