libs.tech: Klayout: tech: sg13g2.lyt: Use empty lef-files

OpenROAD-flow-scripts will search and replace for
<lef-files>.*</lef-files>. Therefore, keep it empty in this syntax.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>

ihp-sg13g2/libs.tech/klayout/tech/sg13g2.lyt

@@ -84,7 +84,7 @@
    <macro-resolution-mode>default</macro-resolution-mode>
    <separate-groups>false</separate-groups>
    <map-file>sg13g2.map</map-file>
-   <lef-files/>
+   <lef-files></lef-files>
   </lefdef>
   <mebes>
    <invert>false</invert>