vsftpd: Add build option to disable utmpx update code

This was modeled after a similar option for Dropbear.

The utmpx code is automatically disabled when compiling with musl,
to avoid a build error due to WTMPX_FILE being undefined. Note that
musl has an empty utmpx implementation, so no functionality is lost
by not calling it.

[Peter: use positive logic]
Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>