REORG: include: move istbuf.h to haproxy/

This one now relies on two files that were already cleaned up and is
only used by buffer.h.
diff --git a/include/common/buffer.h b/include/common/buffer.h
index 6e7adf4..2f5f882 100644
--- a/include/common/buffer.h
+++ b/include/common/buffer.h
@@ -30,7 +30,7 @@
 #include <haproxy/buf.h>
 #include <common/chunk.h>
 #include <import/ist.h>
-#include <common/istbuf.h>
+#include <haproxy/istbuf.h>
 #include <common/memory.h>
 
 #include <proto/activity.h>