Fix build errors when ENABLE_VERBOSE is not set

Fixes #59

When ENABLE_VERBOSE is not set some additional calls to tcpreplay_seterr
are made. When conforming to C99 variadic macros at least one argument is
required after the format string[1]. To avoid this problem update the
calls to tcpreplay_seterr to use a "%s" format string.

[1] - http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html

Signed-off-by: Chris Packham <judge.packham@gmail.com>