lib: thread history funcname shouldn't be constant, it's freed

* thread.h: (struct cpu_thread_history.funcname) malloc gets called on this
  so it should not be const
* thread.c: (cpu_record_print) cast the (const char *) to (char *), this
  function knows what it's doing (free wont be called on it).
diff --git a/lib/thread.h b/lib/thread.h
index b52bc54..937bedb 100644
--- a/lib/thread.h
+++ b/lib/thread.h
@@ -82,7 +82,7 @@
 struct cpu_thread_history 
 {
   int (*func)(struct thread *);
-  const char *funcname;
+  char *funcname;
   unsigned int total_calls;
   struct time_stats
   {