sdc-plugin: Add missing includes.

sdc-plugin/buffers.cc

diff --git a/sdc-plugin/buffers.cc b/sdc-plugin/buffers.cc
index 9c2308a..13177a7 100644
--- a/sdc-plugin/buffers.cc
+++ b/sdc-plugin/buffers.cc
@@ -18,6 +18,9 @@
 #include "buffers.h"
 #include <cassert>
 #include <cmath>
+#include <limits>
+#include <string>
+#include <vector>
 
 const std::vector<std::string> Pll::inputs = {"CLKIN1", "CLKIN2"};
 const std::vector<std::string> Pll::outputs = {"CLKOUT0", "CLKOUT1", "CLKOUT2", "CLKOUT3", "CLKOUT4", "CLKOUT5"};