[SYCL] Add missing test macro for sycl_ext_oneapi_peer_access (#21946)
The test macro for the `sycl_ext_oneapi_peer_access` extension, namely
`SYCL_EXT_ONEAPI_PEER_ACCESS`, is missing. This PR add it to fix the
problem as reported in https://github.com/intel/llvm/issues/21393.
Signed-off-by: Hu, Peisen <peisen.hu@intel.com>