Fix compilation error on Windows (MSYS64)

[homes32]

This PR addresses a build failure encountered when compiling Gpredict on Windows using the MSYS64 environment.

../src/map-selector.c:105:42: error: passing argument 2 of 'g_stat' from incompatible pointer type [-Wincompatible-pointer-types]
  105 |          if (G_UNLIKELY(g_stat(selection, &sb) < 0))
      |                                           ^~~
      |                                           |
      |                                           struct stat *
note: expected 'GStatBuf *' {aka 'struct _stat64 *'} but argument is of type 'struct stat *'

The issue stems from a type mismatch in g_stat calls within src/map-selector.c. While struct stat is standard on Linux, GLib on Windows expects GStatBuf (which maps to struct _stat64). Passing a standard struct stat pointer results in an incompatible-pointer-types error.

Changes

Replaced struct stat with the portable GLib type GStatBuf in create_preview_widget() and update_preview_widget().

This ensures that g_stat receives the correct pointer type.