SuperBuild: skip install export.cmake for libpng

SuperBuild/CMake/External_png.cmake

@@ -24,6 +24,7 @@ ExternalProject_Add(PNG
   -DPNG_TESTS:BOOL=OFF
   -DVERSIONED_OUTPUT_LIB:BOOL=OFF
   -DSKIP_INSTALL_FILES:BOOL=ON
+  -DSKIP_INSTALL_EXPORT:BOOL=ON
   -DSKIP_INSTALL_EXECUTABLES:BOOL=OFF
   ${PNG_SB_CONFIG}
   CMAKE_COMMAND ${SB_CMAKE_COMMAND}