REORG: include: move signal.h to haproxy/signal{,-t}.h

No change was necessary. Include from wdt.c was dropped since unneeded.
diff --git a/src/signal.c b/src/signal.c
index 98ed992..6d01548 100644
--- a/src/signal.c
+++ b/src/signal.c
@@ -14,7 +14,7 @@
 #include <string.h>
 
 #include <haproxy/task.h>
-#include <proto/signal.h>
+#include <haproxy/signal.h>
 #include <proto/log.h>
 
 /* Principle : we keep an in-order list of the first occurrence of all received