gtest: Update path in prep.sh
gtest: Update path in prep.sh

It was not updated when it was relocated to new upstream

Bug: https://jira.iotivity.org/browse/IOT-820
Bug-Tizen: https://bugs.tizen.org/jira/browse/TINF-861
Change-Id: I4b8c2d4e6f398fc5941bff55ac3ab74b376e5e7a
Signed-off-by: Philippe Coval <ff46fec9739d31cce915aabcc3a7a2c00eee4d6c@osg.samsung.com>

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