trace: correct trace_strbuf() parameter type for !HAVE_VARIADIC_MACROS

Reported-by: dev <dev@cor0.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace.c

@@ -216,7 +216,7 @@ void trace_argv_printf(const char **argv, const char *format, ...)
 	va_end(ap);
 }
 
-void trace_strbuf(const char *key, const struct strbuf *data)
+void trace_strbuf(struct trace_key *key, const struct strbuf *data)
 {
 	trace_strbuf_fl(NULL, 0, key, data);
 }