Fix a race between fork/trace_me and first ptrace request

- That first ptrace may fail if ltrace is too quick
diff --git a/execute_program.c b/execute_program.c
index 47f514d..859f32c 100644
--- a/execute_program.c
+++ b/execute_program.c
@@ -89,6 +89,8 @@
 		_exit(1);
 	}
 
+	wait_for_proc(pid);
+
 	debug(1, "PID=%d", pid);
 
 	return pid;