Fix warning from Windows about loss of data in cast-less assignment.

src/utils.h

Index: src/utils.h
===================================================================
--- src/utils.h	(revision 549)
+++ src/utils.h	(working copy)
@@ -448,7 +448,11 @@
 template <typename sourcechar, typename sinkchar>
 static inline void CopyChars(sinkchar* dest, const sourcechar* src, int chars) {
   while (chars--) {
-    *dest++ = *src++;
+    // We have to have a cast here, otherwise MSVC complains about a possible
+    // loss of data when sourcechar is 16 bit and sinkchar is 8 bit.  But we
+    // can't have a reinterpret cast because gcc doesn't like a reinterpret_cast
+    // that doesn't change the type.
+    *dest++ = (sinkchar)(*src++);
   }
 }