Mark DateTimeParseError() noreturn

This avoids a warning from clang 3.2 about an uninitialized variable
'dtype' in date_in().

src/include/utils/datetime.h

@@ -285,7 +285,7 @@ extern int DecodeISO8601Interval(char *str,
 					  int *dtype, struct pg_tm * tm, fsec_t *fsec);
 
 extern void DateTimeParseError(int dterr, const char *str,
-				   const char *datatype);
+				   const char *datatype) __attribute__((noreturn));
 
 extern int	DetermineTimeZoneOffset(struct pg_tm * tm, pg_tz *tzp);