build: Drop pretty option to support older gcovr
build: Drop pretty option to support older gcovr

If using gcovr 2.4 (r2774) just use --xml for reporting
on Ubuntu 14.04

Bug: https://jira.iotivity.org/browse/IOT-1745
Change-Id: Ib9c55ff2efcc90105305b3dcf7af8698a8e022a9
Signed-off-by: Philippe Coval <ff46fec9739d31cce915aabcc3a7a2c00eee4d6c@osg.samsung.com>

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