Set mnemonics for the closest preceding label

Due to a wrong "else" in RealizeTabOrder() implementation, we could set
the mnemonic widget for a previous widget using mnemonics (i.e.
wxStaticText or wxStaticBox) rather than the one closest to the actual
control activated by the mnemonic.

src/gtk/window.cpp

@@ -4786,7 +4786,8 @@ void wxWindowGTK::RealizeTabOrder()
                         }
                     }
                 }
-                else if ( win->GTKWidgetNeedsMnemonic() )
+
+                if ( win->GTKWidgetNeedsMnemonic() )
                 {
                     mnemonicWindow = win;
                 }