fix build when WIN32_LEAN_AND_MEAN is passed on command line

Author: Acly

src/visp/platform.cpp

@@ -1,7 +1,9 @@
 #include "visp/platform.hpp"
 
 #ifdef _WIN32
-#    define WIN32_LEAN_AND_MEAN
+#    ifndef WIN32_LEAN_AND_MEAN
+#       define WIN32_LEAN_AND_MEAN
+#    endif
 #    include <windows.h>
 #else
 #    include <dlfcn.h>