Add SSL_SRV requirement to test

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index c5d0d9a..41dd491 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -2376,6 +2376,7 @@
 requires_config_enabled MBEDTLS_SSL_PROTO_TLS1_2
 requires_config_enabled MBEDTLS_DEBUG_C
 requires_config_enabled MBEDTLS_SSL_CLI_C
+requires_config_enabled MBEDTLS_SSL_SRV_C
 requires_config_enabled MBEDTLS_ECDSA_C
 requires_hash_alg SHA_256
 run_test    "Single supported algorithm sending" \