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

thanks @madebr

include/build_config/SDL_build_config_windows.h

@@ -86,6 +86,7 @@ typedef unsigned int uintptr_t;
 #define HAVE_DXGI_H 1
 #define HAVE_XINPUT_H 1
 #if defined(_WIN32_MAXVER) && _WIN32_MAXVER >= 0x0A00  /* Windows 10 SDK */
+#define HAVE_DXGI1_6_H 1
 #define HAVE_WINDOWS_GAMING_INPUT_H 1
 #endif
 #if defined(_WIN32_MAXVER) && _WIN32_MAXVER >= 0x0602  /* Windows 8 SDK */