Add optional TID definition to the systemd sink send

From the systemd.journal-fields the TID is a user defined
field passed directly from the clients and stored in the
journal. Adding the arguement in the journal send to support
that storage option in the journal.

include/spdlog/sinks/systemd_sink.h

@@ -8,9 +8,11 @@
 #include <spdlog/details/synchronous_factory.h>
 
 #include <array>
+#include <cstdint>
 #ifndef SD_JOURNAL_SUPPRESS_LOCATION
 #    define SD_JOURNAL_SUPPRESS_LOCATION
 #endif
+#include <unistd.h>
 #include <systemd/sd-journal.h>
 
 namespace spdlog {
@@ -75,11 +77,17 @@ class systemd_sink : public base_sink<Mutex>
         {
             // Note: function call inside '()' to avoid macro expansion
             err = (sd_journal_send)("MESSAGE=%.*s", static_cast<int>(length), payload.data(), "PRIORITY=%d", syslog_level(msg.level),
+#ifndef SPDLOG_NO_THREAD_ID
+                "TID=%jd", static_cast<intmax_t>(gettid()),
+#endif
                 "SYSLOG_IDENTIFIER=%.*s", static_cast<int>(syslog_identifier.size()), syslog_identifier.data(), nullptr);
         }
         else
         {
             err = (sd_journal_send)("MESSAGE=%.*s", static_cast<int>(length), payload.data(), "PRIORITY=%d", syslog_level(msg.level),
+#ifndef SPDLOG_NO_THREAD_ID
+                "TID=%jd", static_cast<intmax_t>(gettid()),
+#endif
                 "SYSLOG_IDENTIFIER=%.*s", static_cast<int>(syslog_identifier.size()), syslog_identifier.data(), "CODE_FILE=%s",
                 msg.source.filename, "CODE_LINE=%d", msg.source.line, "CODE_FUNC=%s", msg.source.funcname, nullptr);
         }