Add missing dependency that isn't autodetected

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index f21cfc9..f4c7f6d 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -9127,6 +9127,7 @@
             -S "Ciphersuite is .*-EC.*" \
             -s "dumping 'client hello extensions' (0 bytes)"
 
+requires_config_enabled MBEDTLS_KEY_EXCHANGE_PSK_ENABLED
 requires_gnutls
 run_test    "ClientHello without extensions: PSK" \
             "$P_SRV force_version=tls12 debug_level=3 psk=73776f726466697368" \