Actually include dark mode functions header from implementation

This was forgotten before and resulted in the parent commit not having
any effect, i.e. IsActive() still not being exported as intended.

src/msw/darkmode.cpp

@@ -47,6 +47,8 @@
 #include "wx/msw/dc.h"
 #include "wx/msw/uxtheme.h"
 
+#include "wx/msw/private/darkmode.h"
+
 static const char* TRACE_DARKMODE = "msw-darkmode";
 
 #define DWMWA_USE_IMMERSIVE_DARK_MODE 20