tools: gpio: fix %llu warning in gpio-watch.c

Some platforms, such as mips64, don't map __u64 to long long unsigned
int so using %llu produces a warning:

gpio-watch.c: In function ‘main’:
gpio-watch.c:89:30: warning: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 4 has type ‘__u64’ {aka ‘long unsigned int’} [-Wformat=]
   89 |    printf("line %u: %s at %llu\n",
      |                           ~~~^
      |                              |
      |                              long long unsigned int
      |                           %lu
   90 |           chg.info.offset, event, chg.timestamp_ns);
      |                                   ~~~~~~~~~~~~~~~~
      |                                      |
      |                                      __u64 {aka long unsigned int}

Replace the %llu with PRIu64 and cast the argument to uint64_t.

Fixes: 33f0c47b

 ("tools: gpio: implement gpio-watch")
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>