Fix format-string warning

Cast size_t to (unsigned long) and print it with the %lu format
string, which is more portable than C99's explict %zu for size_t.

This truncates on Windows x64 but otherwise works on 32 and 64 bit
platforms. In practice the stats file is unlikely to be so large.

Change-Id: I0432b3acf85fc6ba4ad50640942e1ca4614b21cb

vpxenc.c

@@ -163,8 +163,8 @@ int stats_open_file(stats_io_t *stats, const char *fpf, int pass)
 
         if (!stats->buf.buf)
         {
-            fprintf(stderr, "Failed to allocate first-pass stats buffer (%d bytes)\n",
-                    stats->buf_alloc_sz);
+            fprintf(stderr, "Failed to allocate first-pass stats buffer (%lu bytes)\n",
+                    (unsigned long)stats->buf_alloc_sz);
             exit(EXIT_FAILURE);
         }