Use is_pod, add <system_error> include for TcpInfo

Summary:
- `std::is_pod_v` is only available in C++17, shifting to `std::is_pod`
- `std::errc` needs the `system_error` header; missing header wasn't causing an error on most platforms because it is included indirectly

Differential Revision: D29355681

fbshipit-source-id: e035c3f4ffac9d2c6f0d8ec511f7e0ea8544ba80

folly/net/TcpInfo.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <chrono>
+#include <system_error>
 
 #include <folly/Expected.h>
 #include <folly/Optional.h>
@@ -229,7 +230,7 @@ struct TcpInfo {
    */
   template <typename T1, typename T2>
   static size_t constexpr getFieldOffset(T1 T2::*field) {
-    static_assert(std::is_pod_v<T1>);
+    static_assert(std::is_pod<T1>());
     constexpr T2 dummy{};
     return size_t(&(dummy.*field)) - size_t(&dummy);
   }