tcpsockets: Don't pull in platform-specific headers in headers. (#26)

This pollutes the namespace for all users of these headers.
This is particularly an issue on Windows.

src/main/native/cpp/tcpsockets/SocketError.cpp

@@ -8,13 +8,23 @@
 #include "tcpsockets/SocketError.h"
 
 #ifdef _WIN32
+#include <WinSock2.h>
 #include <windows.h>
 #else
+#include <errno.h>
 #include <string.h>
 #endif
 
 namespace wpi {
 
+int SocketErrno() {
+#ifdef _WIN32
+  return WSAGetLastError();
+#else
+  return errno;
+#endif
+}
+
 std::string SocketStrerror(int code) {
 #ifdef _WIN32
   LPSTR errstr = nullptr;