Fix CMake error with MSVC compiler (#1794)

Author: KindDragon

src/libOpenImageIO/CMakeLists.txt

@@ -155,7 +155,9 @@ if (WIN32)
     target_link_libraries (OpenImageIO psapi.lib)
 endif ()
 
-add_dependencies (OpenImageIO "${VISIBILITY_MAP_FILE}")
+if (VISIBILITY_MAP_FILE)
+    add_dependencies (OpenImageIO "${VISIBILITY_MAP_FILE}")
+endif ()
 
 if (USE_EXTERNAL_PUGIXML)
     target_link_libraries (OpenImageIO ${PUGIXML_LIBRARIES})