[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);
          |                                    ^

Author: mstorsjo

lldb/tools/lldb-mcp/lldb-mcp.cpp

@@ -17,6 +17,10 @@
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/WithColor.h"
 
+#if defined(_WIN32)
+#include <fcntl.h>
+#endif
+
 using namespace lldb_protocol::mcp;
 
 using lldb_private::File;