trace.c: mark file-local function static

Signed-off-by: NVasyl' Vavrychuk <vvavrychuk@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

trace.c

@@ -25,7 +25,7 @@
 #include "cache.h"
 #include "quote.h"
 
-void do_nothing(size_t unused)
+static void do_nothing(size_t unused)
 {
 }