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/syscall.c b/syscall.c
index 28bdb66..ca721ab 100644
--- a/syscall.c
+++ b/syscall.c
@@ -2523,8 +2523,7 @@
 	}
 
 	if (cflag) {
-		struct timeval t = tv;
-		count_syscall(tcp, &t);
+		count_syscall(tcp, &tv);
 		if (cflag == CFLAG_ONLY_STATS) {
 			goto ret;
 		}