patch: change magic constants to DEFINE value for readability.

in src/backend/utils/misc/tzparser.c

It uses 60 * 60 to represent SECS_PER_HOUR.

and It is already define in other files.

so I think using SECS_PER_HOUR is more clear for readability.

and I attached patch.(it just change 60 * 60 to SECS_PER_HOUR)

What do you think?