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/include/os.h b/include/os.h
index 8665f70..950433d 100644
--- a/include/os.h
+++ b/include/os.h
@@ -11,6 +11,8 @@
 #ifndef __OS_H__
 #define __OS_H__
 
+#include <linux/types.h>
+
 struct sandbox_state;
 
 /**
@@ -116,7 +118,7 @@
  *
  * \return A monotonic increasing time scaled in nano seconds
  */
-u64 os_get_nsec(void);
+uint64_t os_get_nsec(void);
 
 /**
  * Parse arguments and update sandbox state.