sdjournal: fix unused parameter error

Seeing the following with higher warning levels:

    ../phosphor-logging/sdjournal.hpp:33:47: error: unused parameter
            ‘fmt’ [-Werror=unused-parameter]
        33 |     virtual int journal_send_call(const char* fmt)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5626b8fe7c1f17bc08763f69ae040891d22cf1e9
diff --git a/phosphor-logging/sdjournal.hpp b/phosphor-logging/sdjournal.hpp
index 78820fb..975a52e 100644
--- a/phosphor-logging/sdjournal.hpp
+++ b/phosphor-logging/sdjournal.hpp
@@ -30,7 +30,7 @@
      * @return an int meant to be intepreted by the journal_send caller during
      * testing.
      */
-    virtual int journal_send_call(const char* fmt)
+    virtual int journal_send_call(const char* /*fmt*/)
     {
         return 0;
     };