Revert "Revert part of commit dddf4cd."

This reverts commit c114229.
Commit 1408d5d should make it
safe to include these headers in the natural order.

Author: tglsfdc

src/interfaces/ecpg/pgtypeslib/dt_common.c

@@ -7,9 +7,9 @@
 #include <math.h>
 
 #include "common/string.h"
-#include "pgtypeslib_extern.h"
 #include "dt.h"
 #include "pgtypes_timestamp.h"
+#include "pgtypeslib_extern.h"
 
 const int	day_tab[2][13] = {
 	{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31, 0},