qtest: avoid a warning with RTC test

Avoid this warning on OpenBSD:
  CC    tests/rtc-test.o
/src/qemu/tests/rtc-test.c: In function 'check_time':
/src/qemu/tests/rtc-test.c:171: warning: format '%ld' expects type 'long int', but argument 2 has type 'time_t'
/src/qemu/tests/rtc-test.c:173: warning: format '%ld' expects type 'long int', but argument 2 has type 'time_t'

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
