[SYCL][LIT] Fix mock_opencl.cpp warning (#21814) (#22127)
Warning is
```
/__w/llvm/llvm/src/sycl/unittests/mock_opencl/mock_opencl.cpp:8:15: error: variable 'eventRetains' set but not used [-Werror,-Wunused-but-set-global]
8 | static size_t eventRetains = 0;
```
Seems like a copy-and-paste bug, use the correct variable.
Signed-off-by: Nick Sarnie <nick.sarnie@intel.com>
Co-authored-by: Nick Sarnie <nick.sarnie@intel.com>
