Constify count_syscall function

* count.c (count_syscall): Add const qualifier to timeval argument and
rename it.  Store the wall clock time spent while in syscall in separate
timeval variable.
* defs.h (count_syscall): Update prototype.
* syscall.c (trace_syscall_exiting): Update count_syscall invocation.
diff --git a/defs.h b/defs.h
index 22e847b..002271a 100644
--- a/defs.h
+++ b/defs.h
@@ -587,7 +587,7 @@
 extern void qualify(const char *);
 extern void print_pc(struct tcb *);
 extern int trace_syscall(struct tcb *);
-extern void count_syscall(struct tcb *, struct timeval *);
+extern void count_syscall(struct tcb *, const struct timeval *);
 extern void call_summary(FILE *);
 
 #if defined(AVR32) \