transaction: fix -Wconversion c++ warning

Building sdbusplus by meson subproject as a part of external project
contains compile warning which can be interpreted as compile error (if
external project defines both `-Werror` and `-Wconversion`)

Changing the `Transaction::time` property type to the appropriate
`std::time_t` will solve the issue.

Change-Id: I9ab42472ff361aab9487c73d7804531151986e4c
Signed-off-by: Igor Kononenko <i.kononenko.e@gmail.com>
diff --git a/src/server/transaction.cpp b/src/server/transaction.cpp
index bfd902a..671116d 100644
--- a/src/server/transaction.cpp
+++ b/src/server/transaction.cpp
@@ -88,7 +88,7 @@
 size_t hash<sdbusplus::server::transaction::details::Transaction>::operator()(
     const sdbusplus::server::transaction::details::Transaction& t) const
 {
-    auto hash1 = std::hash<int>{}(t.time);
+    auto hash1 = std::hash<std::time_t>{}(t.time);
     auto hash2 = std::hash<std::thread::id>{}(t.thread);
 
     // boost::hash_combine() algorithm.