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/protocol_buffers.h b/include/proto/protocol_buffers.h
index 3e1571a..16ccee5 100644
--- a/include/proto/protocol_buffers.h
+++ b/include/proto/protocol_buffers.h
@@ -24,8 +24,8 @@
 
 #include <inttypes.h>
 #include <haproxy/arg-t.h>
+#include <haproxy/sample-t.h>
 #include <types/protocol_buffers.h>
-#include <proto/sample.h>
 
 #define PBUF_VARINT_DONT_STOP_BIT       7
 #define PBUF_VARINT_DONT_STOP_BITMASK  (1 << PBUF_VARINT_DONT_STOP_BIT)