Fix compilation of EnumerateLoadedModules() call for MinGW64 4.5.

Fix compilation of EnumerateLoadedModules() call for MinGW64 4.5.

Our logic for detection of whether the first argument of
PENUMLOADED_MODULES_CALLBACK is const or not didn't work for MinGW64 4.5.

Replace it with a less error-prone (even if more ugly) version which simply
casts the callback to the type expected by the function.

Closes #12465.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65535 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775