Don't use spi_priv.h in plpython.

There may once have been a reason to violate modularity like that,
but it doesn't appear that there is anymore.

src/pl/plpython/plpy_spi.c

@@ -9,7 +9,7 @@
 #include "access/htup_details.h"
 #include "access/xact.h"
 #include "catalog/pg_type.h"
-#include "executor/spi_priv.h"
+#include "executor/spi.h"
 #include "mb/pg_wchar.h"
 #include "parser/parse_type.h"
 #include "utils/memutils.h"