login: Fix unused variables in login.c if _HAVE_UT_TV isn't defined

Wrap around the variable declaration for `tv' with ifdef so the compilers
don't warn about unused variables if we're compiling without UT_TV. This
happens with the musl libc, since it doesn't define _HAVE_UT_TV, even
though it _does_ have the ut_tv field in the utmp struct.

Signed-off-by: Will Johansson <will.johansson@gmail.com>