cmake: remove GetDpiForMonitor() assignment from shellscalingapi.h check

It always fails, because we don't set the needed link library, i.e. shcore.
Besides, SDL3.dll doesn't link to shcore.dll, it links to it at runtime, so
it is unnecessary to check for that symbol here.

Reference issue: https://github.com/libsdl-org/SDL/pull/13904

CMakeLists.txt

@@ -2207,12 +2207,7 @@ elseif(WINDOWS)
   check_include_file(mmdeviceapi.h HAVE_MMDEVICEAPI_H)
   check_include_file(audioclient.h HAVE_AUDIOCLIENT_H)
   check_include_file(sensorsapi.h HAVE_SENSORSAPI_H)
-  check_c_source_compiles("
-    #include <shellscalingapi.h>
-    static void *ptr = GetDpiForMonitor;
-    int main (int argc, char **argv) { return 0; }
-    " HAVE_SHELLSCALINGAPI_H
-  )
+  check_include_file(shellscalingapi.h HAVE_SHELLSCALINGAPI_H)
   check_c_source_compiles("
     #include <windows.h>
     #include <mfapi.h>