Add compile-guard for < TLS1.2 path in server-side ssl_pick_cert()

Minor code-size optimization along the way.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index f661d11..f00e44b 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -862,6 +862,8 @@
             continue;
         }
 
+#if defined(MBEDTLS_SSL_PROTO_SSL3) || defined(MBEDTLS_SSL_PROTO_TLS1) || \
+    defined(MBEDTLS_SSL_PROTO_TLS1_1)
         /*
          * Try to select a SHA-1 certificate for pre-1.2 clients, but still
          * present them a SHA-higher cert rather than failing if it's the only
@@ -890,6 +892,9 @@
                 continue;
             }
         }
+#endif /* MBEDTLS_SSL_PROTO_TLS1   ||
+          MBEDTLS_SSL_PROTO_TLS1_1 ||
+          MBEDTLS_SSL_PROTO_SSL3 */
 
         /* If we get there, we got a winner */
         break;