spec: make dist tag optional (#659)

Technically the %{dist} RPM macro is optional, so Fedora's convention is
to use a question mark in the Release field.

https://docs.fedoraproject.org/en-US/packaging-guidelines/DistTag/

Practically speaking, %{dist} is almost always defined, so the question
mark is not necessary, but this silences a warning in one of Red Hat's
internal linters.

tcmu-runner.spec

@@ -39,7 +39,7 @@ Version:       1.5.4
 URL:           https://github.com/open-iscsi/tcmu-runner
 
 #%define _RC
-Release:       %{?_RC:%{_RC}}%{dist}
+Release:       %{?_RC:%{_RC}}%{?dist}
 BuildRoot:     %(mktemp -udp %{_tmppath}/%{name}-%{version}%{?_RC:-%{_RC}})
 Source:       %{name}-%{version}%{?_RC:-%{_RC}}.tar.gz
 ExclusiveOS:   Linux