Only define get_zone() and get_gmtoff() if needed (#1193)

Only define the get_zone() and get_gmtoff() private functions in the
time module if these functions are needed to initialize the module.

The change fixes the following warnings on AIX:

Modules/timemodule.c:1175:1: warning: 'get_gmtoff' defined but not used [-Wunused-function]
Modules/timemodule.c:1164:1: warning: 'get_zone' defined but not used [-Wunused-function]