REORG: include: move time.h from common/ to haproxy/

This one is included almost everywhere and used to rely on a few other
.h that are not needed (unistd, stdlib, standard.h). It could possibly
make sense to split it into multiple parts to distinguish operations
performed on timers and the internal time accounting, but at this point
it does not appear much important.
diff --git a/include/proto/backend.h b/include/proto/backend.h
index 5160492..f702be9 100644
--- a/include/proto/backend.h
+++ b/include/proto/backend.h
@@ -23,7 +23,7 @@
 #define _PROTO_BACKEND_H
 
 #include <haproxy/api.h>
-#include <common/time.h>
+#include <haproxy/time.h>
 
 #include <types/backend.h>
 #include <types/proxy.h>