core: Fix signed vs unsigned compare

core: Fix signed vs unsigned compare

__time_t is not a portable data type, and can cause sign mismatch on
some compares.

Fixes:
  CC       src/bluetoothd-device.o
src/device.c: In function ‘device_is_name_resolve_allowed’:
src/device.c:4092:17: error: comparison of integer expressions of
different signedness: ‘__time_t’ {aka ‘long int’} and
‘long unsigned int’ [-Werror=sign-compare]
  if (now.tv_sec >= device->name_resolve_failed_time +
                 ^~
cc1: all warnings being treated as errors

Signed-off-by: Manika Shrivastava <manika.sh@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>