liststore.hg: Rename a local variable (fix `-Dwarnings=max` on Visual Studio)

Hi,

From the commit message:

In the templatized implementation of std::pair<bool, unsigned int> ListStore<T_item>::find, replace result with find_result for what g_list_store_find_with_equal_func_full() returns, to avoid a compiler warning when building the giomm_listmodel test program when -Dwarnings=max is specified, for Visual Studio builds, as we are using a variable result in a rather global scrope there[1].

This will help fix 'meson dist' on Visual Studio builds.

[1]: https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-4-c4459

With blessings, thank you!