benchmark: fix build

Fixes the following error [1]:

make -C /home/travis/default/build/../optee_benchmark HOST_CROSS_COMPILE="/usr/bin/ccache /home/travis/default/build/../toolchains/aarch32/bin/arm-linux-gnueabihf-" TEEC_EXPORT=/home/travis/default/build/../optee_client/out/export TEEC_INTERNAL_INCLUDES=/home/travis/default/build/../optee_client/libteec
make: *** /home/travis/default/build/../optee_benchmark: No such file or directory.  Stop.

[1] https://travis-ci.org/OP-TEE/build/builds/237729031

 job 216.1

Fixes: 27edfc78 ("OP-TEE Benchmark")
Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>
Tested-by: Jerome Forissier <jerome.forissier@linaro.org. (HiKey)
[jf: update commit description]
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>