libgimpbase: in gimp_stack_trace_print(), clear ptrace permission on exit

Clear the ptrace permission given to the child after it terminates,
so that a future process that happens to have the same PID the
child had can't ptrace us.

libgimpbase/gimputils.c

@@ -1252,6 +1252,11 @@ gimp_stack_trace_print (const gchar *prog_name,
 
       waitpid (pid, &status, 0);
 
+#ifdef PR_SET_PTRACER
+      /* Clear ptrace permission set above */
+      prctl (PR_SET_PTRACER, 0, 0, 0, 0);
+#endif
+
       while ((read_n = read (out_fd[0], buffer, 256)) > 0)
         {
           /* It's hard to know if the debugger was found since it