Fix windows build.

This fixes the lack of uint32_t definition in source/val/decoration.h.
diff --git a/source/val/decoration.h b/source/val/decoration.h
index ce74990..e6e85ea 100644
--- a/source/val/decoration.h
+++ b/source/val/decoration.h
@@ -15,6 +15,7 @@
 #ifndef LIBSPIRV_VAL_DECORATION_H_
 #define LIBSPIRV_VAL_DECORATION_H_
 
+#include <cstdint>
 #include <unordered_map>
 #include <vector>