FindwxWidgets: Fix finding both release and debug libs

In commit fe54989f (FindwxWidgets: Add support for wxQt, 2019-11-12,
v3.16.0-rc4~12^2~1) the internal `WX_FIND_LIBS` macro gained an argument
but not all call sites were updated.  Update the missing one now.

Fixes: #20005
14 jobs for FindwxWidgets-rel-and-dbg
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