check dxgi1_6.h before enabling sdr/hdr code in SDL_windowsmodes.c

thanks @madebr

include/build_config/SDL_build_config_wingdk.h

@@ -37,6 +37,7 @@
 #define HAVE_DSOUND_H 1
 /* No SDK version checks needed for these because the SDK has to be new. */
 #define HAVE_DXGI_H 1
+#define HAVE_DXGI1_6_H 1
 #define HAVE_XINPUT_H 1
 #define HAVE_WINDOWS_GAMING_INPUT_H 1
 #define HAVE_D3D11_H 1