This fixes a compilation error in wxMSW private fonts support
implementation, which compares wxString with a wide string but, unlike a
local fix there, makes sense more broadly and should reduce the
likelihood of similar errors in the future.
It also makes comparisons with narrow C strings more efficient in the
default, Unicode, build by using wxString::Cmp() method instead of
creating a temporary wxString, as was done before.
See #18172.
d390bee8a2