fix: flags for ngtcp2 on IBM i

similar to aix IBM i does not support certain network flags
we are getting the following build errors:

```
../deps/ngtcp2/ngtcp2/examples/shared.cc:68:63: error: 'IPTOS_ECN_MASK' was not declared in this scope
   68 |         return *reinterpret_cast<uint8_t *>(CMSG_DATA(cmsg)) & IPTOS_ECN_MASK;
      |                                                               ^~~~~~~~~~~~~~
../deps/ngtcp2/ngtcp2/examples/shared.cc:80:22: error: 'IPTOS_ECN_MASK' was not declared in this scope
   80 |         return tos & IPTOS_ECN_MASK;
      |                      ^~~~~~~~~~~~~~
```

Author: sravani1510

deps/ngtcp2/ngtcp2.gyp

@@ -272,7 +272,7 @@
         'HAVE_NETINET_IP_H',
       ],
       'conditions': [
-        ['OS=="aix" or OS=="win"', {
+        ['OS=="aix" or OS=="win" or OS=="os400"', {
           # AIX does not support some of the networking features used in
           # the test server. Windows also lacks the Unix-specific headers
           # and system calls required by the ngtcp2 examples.
@@ -330,7 +330,7 @@
         'HAVE_NETINET_IP_H',
       ],
       'conditions': [
-        ['OS=="aix" or OS=="win"', {
+        ['OS=="aix" or OS=="win" OS=="os400"', {
           # AIX does not support some of the networking features used in
           # the test client. Windows also lacks the Unix-specific headers
           # and system calls required by the ngtcp2 examples.