fix: define _DEBUG macro to 1 on redefinition (pybind#5639)

According to the msvc documentation, the macro should be defined to 1:
https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros

Author: tobiasleibner

include/pybind11/conduit/wrap_include_python_h.h

@@ -50,7 +50,7 @@
 #endif
 
 #if defined(PYBIND11_DEBUG_MARKER)
-#    define _DEBUG
+#    define _DEBUG 1
 #    undef PYBIND11_DEBUG_MARKER
 #endif