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

This one is particularly tricky to move because everyone uses it
and it depends on a lot of other types. For example it cannot include
arg-t.h and must absolutely only rely on forward declarations to avoid
dependency loops between vars -> sample_data -> arg. In order to address
this one, it would be nice to split the sample_data part out of sample.h.
diff --git a/include/proto/spoe.h b/include/proto/spoe.h
index 584b38a..d9e9098 100644
--- a/include/proto/spoe.h
+++ b/include/proto/spoe.h
@@ -23,11 +23,10 @@
 #define _PROTO_SPOE_H
 
 #include <haproxy/intops.h>
+#include <haproxy/sample-t.h>
 
 #include <types/spoe.h>
 
-#include <proto/sample.h>
-
 
 /* Encode a buffer. Its length <len> is encoded as a varint, followed by a copy
  * of <str>. It must have enough space in <*buf> to encode the buffer, else an