tmux: bump to version 2.1

Add patch to fix musl build by removing inclusion of sys/cdefs.h.
Patch was backported from upstream master and can be dropped in the
next version bump.

Update homepage link

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>