Update module list for ELM_MODULES, required for date/time settings

Update module list for ELM_MODULES, required for date/time settings

The module "datetime_input_ctxpopup" is required for the
"datetime/api" slot so that the date/time settings in the Settings
application will display correctly and function properly.

This commit simply appends the missing module/slot entry to
ELM_MODULES in the systemd user session environment file
(/etc/sysconfig/tizen-mobile-ui); by doing this, Elementary will load
the missing module for the user session.

Change-Id: I97432d74876f9cbd05cd0632d3befe60b7cf931e
Signed-off-by: Patrick McCarty <patrick.mccarty@linux.intel.com>