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

thanks @madebr

CMakeLists.txt

@@ -1911,6 +1911,7 @@ elseif(WINDOWS)
   endif()
 
   # headers needed elsewhere
+  check_include_file(dxgi1_6.h HAVE_DXGI1_6_H)
   check_include_file(tpcshrd.h HAVE_TPCSHRD_H)
   check_include_file(roapi.h HAVE_ROAPI_H)
   check_include_file(mmdeviceapi.h HAVE_MMDEVICEAPI_H)