Revert part of commit dddf4cdc.

The commit dddf4cdc tries to ensure that the Postgres header file
inclusions are in order based on their ASCII value.  However, in one of
the case there is a header file dependency due to which we can't maintain
such order.

Author: Amit Kapila
Discussion: https://postgr.es/m/E1iNpHW-000855-1u@gemulon.postgresql.org

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},