[lldb-mcp] Fix building for Windows
This fixes build errors like these:
lldb/tools/lldb-mcp/lldb-mcp.cpp:45:41: error: use of undeclared identifier '_O_BINARY'
45 | int result = _setmode(fileno(stdout), _O_BINARY);
| ^
lldb/tools/lldb-mcp/lldb-mcp.cpp:47:36: error: use of undeclared identifier '_O_BINARY'
47 | result = _setmode(fileno(stdin), _O_BINARY);
| ^
