BUILD: pools: fix build with DEBUG_FAIL_ALLOC

Amaury noticed that I managed to break the build of DEBUG_FAIL_ALLOC
for the second time with 207c09509 ("MINOR: pools: move the fault
injector to __pool_alloc()"). The joy of endlessly reworking patch
sets... No backport is needed, that was in the just merged cleanup
series.
diff --git a/include/haproxy/pool.h b/include/haproxy/pool.h
index 54ec933..a7d77e4 100644
--- a/include/haproxy/pool.h
+++ b/include/haproxy/pool.h
@@ -63,6 +63,7 @@
 void create_pool_callback(struct pool_head **ptr, char *name, unsigned int size);
 void *pool_destroy(struct pool_head *pool);
 void pool_destroy_all();
+int mem_should_fail(const struct pool_head *pool);
 
 
 #ifdef CONFIG_HAP_POOLS
diff --git a/src/pool.c b/src/pool.c
index 88c579b..7009502 100644
--- a/src/pool.c
+++ b/src/pool.c
@@ -38,7 +38,6 @@
 
 #ifdef DEBUG_FAIL_ALLOC
 static int mem_fail_rate = 0;
-static int mem_should_fail(const struct pool_head *);
 #endif
 
 /* Try to find an existing shared pool with the same characteristics and