iwlwifi: mvm: add missing include

This fixes the build for powerpc:

After merging the final tree, today's linux-next build (powerpc allyesconfig)
failed like this:

drivers/net/wireless/iwlwifi/mvm/debugfs.c: In function 'iwl_dbgfs_fw_error_dump_release':
drivers/net/wireless/iwlwifi/mvm/debugfs.c:161:2: error: implicit declaration of function 'vfree' [-Werror=implicit-function-declaration]
  vfree(file->private_data);
  ^

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>