BUILD: resolvers: include tools.h

Many functions from tools.h are called there but it was inherited via others.
diff --git a/src/resolvers.c b/src/resolvers.c
index 44e6eb5..1a05ed3 100644
--- a/src/resolvers.c
+++ b/src/resolvers.c
@@ -43,6 +43,7 @@
 #include <haproxy/tcp_rules.h>
 #include <haproxy/ticks.h>
 #include <haproxy/time.h>
+#include <haproxy/tools.h>
 #include <haproxy/vars.h>