app-misc/tmux: rely on libutempter

app-misc/tmux: rely on libutempter

If utempter.h is installed, tmux unconditionally links against
libutempter, and there isn't a configure switch to disable it. As such,
systems that already had installed libutempter (for use with, say, mosh)
found that there would be issues when libutempter was removed, since it
wasn't an explicit dependency of this package. Meanwhile, there were
issues with the other utempter (non-lib prefix), causing that to be a
blocker. So, the fix for both of these at once is to replace the blocker
against utempter with a dependency on libutempter, because libutempter
already blocks utempter. This simplifes things greatly.

Fixes: 566990
Fixes: 560652
Signed-off-by: Jason A. Donenfeld <zx2c4@gentoo.org>