ecore_time.c: do not mess with the representation of clockid_t.

ecore_time.c: do not mess with the representation of clockid_t.

Clockid_t should be used as an opaque type. Some platform might want
to (and even do, e.g. DragonFlyBSD) declare clockid_t as an unsigned.

On such platform, testing the sign of clockid_t is never false, and
assigning it a negative value is an UB, which makes this code unlikely to
work as intended. Fixes black window on dragonfly!

Thanks to gcc for spotting this.

  CC       lib/ecore/lib_ecore_libecore_la-ecore_time.lo
  In file included from ../src/lib/eina/Eina.h:215:0,
                   from lib/ecore/Ecore.h:304,
                   from lib/ecore/ecore_time.c:18:
                   lib/ecore/ecore_time.c: In function 'ecore_time_get':

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>