BUILD: payload: include tools.h in payload.c

It needs it for memprintf() but used to inherit it via other include files.
diff --git a/src/payload.c b/src/payload.c
index 32e5456..7dd08c8 100644
--- a/src/payload.c
+++ b/src/payload.c
@@ -23,6 +23,7 @@
 #include <haproxy/pattern.h>
 #include <haproxy/payload.h>
 #include <haproxy/sample.h>
+#include <haproxy/tools.h>
 
 
 /************************************************************************/