Add a missing dollar sign

Modules/FindGTK.cmake

@@ -157,7 +157,7 @@ IF(UNIX)
     SET( GTK_GL_FOUND "YES" )
     SET( GTK_INCLUDE_DIR  ${GTK_INCLUDE_DIR}
                            ${GTK_gtkgl_INCLUDE_PATH} )
-    SET( GTK_LIBRARIES  {GTK_gtkgl_LIBRARY} ${GTK_LIBRARIES} )
+    SET( GTK_LIBRARIES  ${GTK_gtkgl_LIBRARY} ${GTK_LIBRARIES} )
     MARK_AS_ADVANCED(
       GTK_gtkgl_LIBRARY
       GTK_gtkgl_INCLUDE_PATH