common.mk: Fix build error with CFG_TEE_BENCHMARK

Steps to reproduce:
1)cd into the QEMU build directory
2)make CFG_TEE_BENCHMARK=y run

We end up in a compile error as mentioned in
https://github.com/OP-TEE/build/issues/489 and
https://github.com/linaro-swg/optee_benchmark/issues/17

Turns out the path to benchmark sources wasn't properly
defined and hence "BR2_PACKAGE_OPTEE_BENCHMARK_EXT_SITE"
was incorrect.

Signed-off-by: Sadiq Hussain <sadiq.muchumarri@intel.com>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/common.mk b/common.mk
index 2dd220c..a5b92b9 100644
--- a/common.mk
+++ b/common.mk
@@ -30,6 +30,7 @@
 BUILD_PATH			?= $(ROOT)/build
 LINUX_PATH			?= $(ROOT)/linux
 UBOOT_PATH			?= $(ROOT)/u-boot
+BENCHMARK_APP_PATH		?= $(ROOT)/optee_benchmark
 OPTEE_OS_PATH			?= $(ROOT)/optee_os
 OPTEE_CLIENT_PATH		?= $(ROOT)/optee_client
 OPTEE_TEST_PATH			?= $(ROOT)/optee_test