BUILD: wdt: include signal-t.h

WDT_SIG is used there, thus signal-t.h is required. Currently it's
retrieved by accident through global.h.
diff --git a/src/wdt.c b/src/wdt.c
index 18836ff..3c0b3d1 100644
--- a/src/wdt.c
+++ b/src/wdt.c
@@ -16,6 +16,7 @@
 #include <haproxy/debug.h>
 #include <haproxy/errors.h>
 #include <haproxy/global.h>
+#include <haproxy/signal-t.h>
 #include <haproxy/thread.h>
 #include <haproxy/tools.h>