Enable -Wmissing-braces in openr/PACKAGE

Summary:
This diff enables the titular warning flag for the directory in question. Further details are in [this workplace post](https://fb.workplace.com/permalink.php?story_fbid=pfbid02XaWNiCVk69r1ghfvDVpujB8Hr9Y61uDvNakxiZFa2jwiPHscVdEQwCBHrmWZSyMRl&id=100051201402394).

This is a low-risk diff. There are **no run-time effects** and the diff has already been observed to compile locally. **If the code compiles, it works; test errors are spurious.**

If the diff does not pass, it will be closed automatically.

Reviewed By: palmje

Differential Revision: D53532263

fbshipit-source-id: 51b3e0ea6840de6a8248f0a6aad9db3fe3e86347

openr/nl/tests/NetlinkProtocolSocketTest.cpp

@@ -530,7 +530,9 @@ class NlMessageFixture : public ::testing::Test {
         ifIndexX,
         1));
     struct v6Addr addr6 {
-      0
+      {
+        { 0 }
+      }
     };
     for (uint32_t i = 0; i < count; i++) {
       addr6.u32_addr[0] = htonl(0x50210000 + i);
@@ -1321,7 +1323,9 @@ TEST_F(NlMessageFixture, MaxPayloadExceeded) {
 
   std::vector<NextHop> paths;
   struct v6Addr addr6 {
-    0
+    {
+      { 0 }
+    }
   };
   for (uint32_t i = 0; i < 200; i++) {
     addr6.u32_addr[0] = htonl(0xfe800000 + i);