BUILD: mworker: include tools.h for platforms without unsetenv()

In this case we fall back to my_unsetenv() thus we need tools.h
to avoid a warning.
diff --git a/src/mworker-prog.c b/src/mworker-prog.c
index 75f64c5..75cfc42 100644
--- a/src/mworker-prog.c
+++ b/src/mworker-prog.c
@@ -27,6 +27,7 @@
 #include <haproxy/mworker.h>
 #include <haproxy/task.h>
 #include <haproxy/time.h>
+#include <haproxy/tools.h>
 
 
 static int use_program = 0; /* do we use the program section ? */