buildroot: always declare BR2_PACKAGE_OPTEE_BENCHMARK_SITE

Using the current tip of the buildroot upstream branch, we get the
following error:

 build$ make
 [...]
 [...]br-ext/package/optee_benchmark/optee_benchmark.mk:8: *** optee_benchmark has local site method, but `OPTEE_BENCHMARK_SITE` is not defined.  Stop.

The error does not occur if BR2_PACKAGE_OPTEE_BENCHMARK is selected (i.e.,
"make CFG_TEE_BENCHMARK=y").

It looks like the _SITE variable has to be declared even though the
package is not selected for building.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>