gdb: make find_thread_ptid an inferior method

Make find_thread_ptid (the overload that takes an inferior) a method of
struct inferior.

Change-Id: Ie5b9fa623ff35aa7ddb45e2805254fc8e83c9cd4
Reviewed-By: Tom Tromey <tom@tromey.com>

gdb/inferior.h

@@ -490,6 +490,9 @@ public:
   inline safe_inf_threads_range threads_safe ()
   { return safe_inf_threads_range (this->thread_list.begin ()); }
 
+  /* Find (non-exited) thread PTID of this inferior.  */
+  thread_info *find_thread (ptid_t ptid);
+
   /* Delete all threads in the thread list.  If SILENT, exit threads
      silently.  */
   void clear_thread_list (bool silent);