Add missing TruffleBoundary

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/ssl/SSLSocketBuiltins.java

@@ -413,6 +413,7 @@ Object get(PSSLSocket socket) {
     @GenerateNodeFactory
     abstract static class SelectedAlpnProtocol extends PythonUnaryBuiltinNode {
         @Specialization
+        @TruffleBoundary
         static Object get(PSSLSocket socket) {
             String protocol = socket.getEngine().getApplicationProtocol();
             return protocol != null && !protocol.isEmpty() ? protocol : PNone.NONE;