libgui: Only allow one return from useFenceSync

An #ifdef was causing multiple returns in this function, which was
triggering an unreachable code error on some build targets. This
adds an #else to ensure that only one return is exposed in any given
build.

Cherry pick of I9ed7c3e769bc9094ad31d810944227d1d8302886

Change-Id: Ibddc5706159d168031d1805fcce3eedc068296cf