Append const keyword for verifier in client library #4

Job	Run time
build_attest_all (buildconfigs/key_xor_test_app.sh)	2m 5s
build_attest_all (buildconfigs/oak_containers_kernel.sh)	11m 16s
build_attest_all (buildconfigs/oak_containers_orchestrator.sh)	6m 20s
build_attest_all (buildconfigs/oak_containers_stage1.sh)	3m 41s
build_attest_all (buildconfigs/oak_containers_syslogd.sh)	5m 56s
build_attest_all (buildconfigs/oak_containers_system_image.sh)	12m 35s
build_attest_all (buildconfigs/oak_echo_enclave_app.sh)	2m 16s
build_attest_all (buildconfigs/oak_echo_raw_enclave_app.sh)	2m 4s
build_attest_all (buildconfigs/oak_functions_enclave_app.sh)	2m 17s
build_attest_all (buildconfigs/oak_functions_insecure_enclave_app.sh)	2m 35s
build_attest_all (buildconfigs/oak_restricted_kernel_simple_io_init_rd_wrapper_bin.sh)	2m 48s
build_attest_all (buildconfigs/stage0_bin.sh)	2m 6s
build_attest_all (buildconfigs/oak_orchestrator.sh)	2m 22s
	58m 21s

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