Fix compile guard for static function in ssl

The guard for the definition of the function was different from the guard on
its only use - make it the same.

This has been caught by tests/scripts/key-exchanges.pl. It had not been caught
by this script in earlier CI runs, because previously USE_PSA_CRYPTO was
disabled in the builds used by this script; enabling it uncovered the issue.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index d25dffd..cd28456 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -729,7 +729,7 @@
 #endif
 #endif /* MBEDTLS_SSL_PROTO_TLS1_2 */
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED) && \
+#if defined(MBEDTLS_KEY_EXCHANGE_PSK_ENABLED) && \
     defined(MBEDTLS_USE_PSA_CRYPTO)
 static int ssl_use_opaque_psk( mbedtls_ssl_context const *ssl )
 {
@@ -749,7 +749,7 @@
     return( 0 );
 }
 #endif /* MBEDTLS_USE_PSA_CRYPTO &&
-          MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED */
+          MBEDTLS_KEY_EXCHANGE_PSK_ENABLED */
 
 #if defined(MBEDTLS_SSL_EXPORT_KEYS)
 static mbedtls_tls_prf_types tls_prf_get_type( mbedtls_ssl_tls_prf_cb *tls_prf )