Use g_list_free_full instead of g_list_foreach

Use g_list_free_full instead of g_list_foreach

- Fixes the following compiler warnings in rrd_create.c:
  236:34: warning: cast between incompatible function types from
  'void (*)(void *)' to 'void (*)(void *, void *)'
  [-Wcast-function-type] g_list_foreach( sources, (GFunc)free, NULL );

  1022:44: warning: cast between incompatible function types from
  'void (*)(rrd_file_t *)' {aka 'void (*)(struct rrd_file_t *)'} to
  'void (*)(void *, void *)' [-Wcast-function-type]
  g_list_foreach(sources_rrd_files, (GFunc) cleanup_source_file, NULL);