journal: fix return types inside C wrapper functions

man pages
- https://www.freedesktop.org/software/systemd/man/sd_journal_open.html
- https://www.freedesktop.org/software/systemd/man/sd_journal_add_match.html

Signed-off-by: René Scheibe <rene.scheibe@gmail.com>

Author: darxriggs

sdjournal/journal.go

@@ -59,7 +59,7 @@ package sdjournal
 // void
 // my_sd_journal_close(void *f, sd_journal *j)
 // {
-//   int (*sd_journal_close)(sd_journal *);
+//   void (*sd_journal_close)(sd_journal *);
 //
 //   sd_journal_close = f;
 //   sd_journal_close(j);
@@ -104,7 +104,7 @@ package sdjournal
 // void
 // my_sd_journal_flush_matches(void *f, sd_journal *j)
 // {
-//   int (*sd_journal_flush_matches)(sd_journal *);
+//   void (*sd_journal_flush_matches)(sd_journal *);
 //
 //   sd_journal_flush_matches = f;
 //   sd_journal_flush_matches(j);