textproc/texi2html: fix gettext dependency

While gettext is not an actual runtime dependency (remove in
06d7ae5e1cadb765ad367b412e6366cca866eac5) it is actually a build
dependency, as the build system uses msgexec(1) binary. the buildsystem
does not fail in case of errors, so if msgexec is not present on the
system while building the port it results in corrupted .mo files.