kernel32: fix a compiler warning

[Austin English]

file.c: In function ‘dumpmem’:
file.c:703: warning: format not a string literal and no format arguments

-- 
-Austin
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diff --git a/dlls/kernel32/tests/file.c b/dlls/kernel32/tests/file.c
index e2ad49d..af64611 100644
--- a/dlls/kernel32/tests/file.c
+++ b/dlls/kernel32/tests/file.c
@@ -700,7 +700,7 @@ static void dumpmem(unsigned char* mem, int len) {
             }
         }
         sprintf(buf+ln, "\n");
-        trace(buf);
+        trace("buffer is %s\n",buf);
         ln = 0;
     }
 }