libglusterfs: Define macro GF_PRI_USEC for Linux as well

Change-Id: I073f1f4ead4391d497fbb7603f9ee0257271493b
BUG: 1089172
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/7571
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Anand Avati <avati@redhat.com>