SDC: Add license header to buffers.cc

Signed-off-by: Tomasz Michalak <tmichalak@antmicro.com>

sdc-plugin/buffers.cc

diff --git a/sdc-plugin/buffers.cc b/sdc-plugin/buffers.cc
index 40c7cb2..ba8fd81 100644
--- a/sdc-plugin/buffers.cc
+++ b/sdc-plugin/buffers.cc
@@ -1,3 +1,20 @@
+/*
+ *  yosys -- Yosys Open SYnthesis Suite
+ *
+ *  Copyright (C) 2020  The Symbiflow Authors
+ *
+ *  Permission to use, copy, modify, and/or distribute this software for any
+ *  purpose with or without fee is hereby granted, provided that the above
+ *  copyright notice and this permission notice appear in all copies.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ *  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ *  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ *  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ *  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ *  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ *  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
 #include "buffers.h"
 
 const std::vector<std::string> Pll::inputs = {"CLKIN1", "CLKIN2"};