Move endian utilities to their own file.
diff --git a/source/validate.cpp b/source/validate.cpp
index 341d618..ff66308 100644
--- a/source/validate.cpp
+++ b/source/validate.cpp
@@ -27,6 +27,7 @@
 #include <libspirv/libspirv.h>
 #include "binary.h"
 #include "diagnostic.h"
+#include "endian.h"
 #include "instruction.h"
 #include "opcode.h"
 #include "operand.h"