configs/qemu_arm_vexpress_tz_defconfig: enable C++ in toolchain

As we are about to bump the optee-test package to 3.11.0, which
requires C++ support, let's enable C++ support in the
qemu_arm_vexpress_tz_defconfig, which uses optee-test.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

configs/qemu_arm_vexpress_tz_defconfig

@@ -7,6 +7,9 @@ BR2_ARM_FPU_VFPV3D16=y
 BR2_TARGET_GENERIC_GETTY_PORT="ttyAMA0"
 BR2_ROOTFS_POST_BUILD_SCRIPT="board/qemu/arm-vexpress-tz/post-build.sh"
 
+# optee-test needs toolchain C++
+BR2_TOOLCHAIN_BUILDROOT_CXX=y
+
 # Filesystem
 BR2_TARGET_ROOTFS_CPIO=y
 BR2_TARGET_ROOTFS_CPIO_GZIP=y