Doc: networking: txtimestamp: fix printf format warning

Documentation/networking/timestamping/txtimestamp.c: In function ‘__print_timestamp’:
Documentation/networking/timestamping/txtimestamp.c:99:3: warning: format ‘%ld’ expects argument of type ‘long int’, but argument 3 has type ‘int64_t’ [-Wformat=]
   fprintf(stderr, "  (%+ld us)", cur_ms - prev_ms);

int64_t differs per platform, so a type specifier that differs along
with it is required.

Signed-off-by: Frans Klaver <fransklaver@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>