Fix WM_SYSKEYDOWN not being trapped by IME message handling

This would cause Alt + <something> not being trapped if handled by the system IME.

(cherry picked from commit 2ef005afe52dcbfe3fa3ff2c7227283100dab43e)

src/video/windows/SDL_windowskeyboard.c

@@ -1019,6 +1019,14 @@ bool WIN_HandleIMEMessage(HWND hwnd, UINT msg, WPARAM wParam, LPARAM *lParam, SD
             SDL_DebugIMELog("WM_KEYDOWN normal");
         }
         break;
+    case WM_SYSKEYDOWN:
+        if (wParam == VK_PROCESSKEY) {
+            SDL_DebugIMELog("WM_SYSKEYDOWN VK_PROCESSKEY");
+            trap = true;
+        } else {
+            SDL_DebugIMELog("WM_SYSKEYDOWN normal");
+        }
+        break;
     case WM_INPUTLANGCHANGE:
         SDL_DebugIMELog("WM_INPUTLANGCHANGE");
         IME_InputLangChanged(videodata);