Win32: Fix warning...

...found by clang+llvm-18.1.8-x86_64-pc-windows-msvc on Windows:

```
[283/325] Building CXX object src/CMakeFiles/solvespace.dir/platform/guiwin.cpp.obj
[...]/SolveSpace/git/src/platform/guiwin.cpp:1188:10: warning: 'SetMinContentSize' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
 1188 |     void SetMinContentSize(double width, double height) {
      |          ^
[...]/SolveSpace/git/src\platform/gui.h:252:18: note: overridden virtual function is here
  252 |     virtual void SetMinContentSize(double width, double height) = 0;
      |                  ^
1 warning generated.
```

src/platform/guiwin.cpp

@@ -1185,7 +1185,7 @@ class WindowImplWin32 final : public Window {
         *height = (rc.bottom - rc.top) / pixelRatio;
     }
 
-    void SetMinContentSize(double width, double height) {
+    void SetMinContentSize(double width, double height) override {
         minWidth  = (int)width;
         minHeight = (int)height;