Fix false positive -Wrestrict warning in HttpUserAgent.h with GCC 12 (#12715)

GCC 12 produces a false positive -Wrestrict warning when assigning the
string literal "-" to std::string in HttpUserAgent::set_txn(). Change
to use char literal '-' instead of string literal "-" to avoid the
warning, similar to commit 355a9ee34d.
diff --git a/include/proxy/http/HttpUserAgent.h b/include/proxy/http/HttpUserAgent.h
index 14520ff..4761e4a 100644
--- a/include/proxy/http/HttpUserAgent.h
+++ b/include/proxy/http/HttpUserAgent.h
@@ -178,7 +178,7 @@
     if (auto group{tbs->get_tls_group()}; !group.empty()) {
       m_conn_info.security_group = group;
     } else {
-      m_conn_info.security_group = "-";
+      m_conn_info.security_group = '-';
     }
 
     if (!m_conn_info.tcp_reused) {