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>

tools/gpio/gpio-watch.c

@@ -10,6 +10,7 @@
 #include <ctype.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <inttypes.h>
 #include <linux/gpio.h>
 #include <poll.h>
 #include <stdbool.h>
@@ -86,8 +87,8 @@ int main(int argc, char **argv)
 				return EXIT_FAILURE;
 			}
 
-			printf("line %u: %s at %llu\n",
-			       chg.info.offset, event, chg.timestamp_ns);
+			printf("line %u: %s at %" PRIu64 "\n",
+			       chg.info.offset, event, (uint64_t)chg.timestamp_ns);
 		}
 	}