sandbox: Use uint64_t instead of u64 for time

The uint64_t type is defined in linux/types.h, so is safer than u64, which
is not actually a Linux type.

Change-Id: Ifc9a369e6543250c49117b8d3cb3a676eee43e04
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/os.c b/arch/sandbox/cpu/os.c
index c2e5f57..db66fd3 100644
--- a/arch/sandbox/cpu/os.c
+++ b/arch/sandbox/cpu/os.c
@@ -8,6 +8,7 @@
 #include <fcntl.h>
 #include <getopt.h>
 #include <stdio.h>
+#include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
 #include <termios.h>
@@ -136,7 +137,7 @@
 	usleep(usec);
 }
 
-u64 __attribute__((no_instrument_function)) os_get_nsec(void)
+uint64_t __attribute__((no_instrument_function)) os_get_nsec(void)
 {
 #if defined(CLOCK_MONOTONIC) && defined(_POSIX_MONOTONIC_CLOCK)
 	struct timespec tp;