Move journal sync function to util.cpp

Move it out of log_manager.cpp so other code will be able to call it.

The code itself wasn't changed, though a comment was modified, as it is
now a utility function it may no longer just be called when an error log
is created.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I33a20c1a68059fd50678464776e88168ce93d030
diff --git a/util.hpp b/util.hpp
index c348916..0ac3878 100644
--- a/util.hpp
+++ b/util.hpp
@@ -16,4 +16,11 @@
  */
 std::optional<std::string> getOSReleaseValue(const std::string& key);
 
+/**
+ * @brief Synchronize unwritten journal messages to disk.
+ * @details This is the same implementation as the systemd command
+ *          "journalctl --sync".
+ */
+void journalSync();
+
 } // namespace phosphor::logging::util