Fix compile error with UTF8 unicode wxWidgets build

Fixes https://gitlab.com/kicad/code/kicad/issues/10210

(Cherry-picked from bd828f598c)

include/trace_helpers.h

@@ -242,7 +242,7 @@ public:
     void DoTraceUtf8( const wxString aWhat, const wxChar* aFmt, ... )
     {
         va_list argptr;
-        va_start( argptr, format );
+        va_start( argptr, aFmt );
         traceV( aWhat, aFmt, argptr );
         va_end( argptr );
     }