Add a cast to make x64 build on windows happy.

Originally committed to SVN as r5155.

aegisub/libaegisub/common/charset_ucd.cpp

@@ -40,7 +40,7 @@ UCDetect::UCDetect(const std::string &file): nsUniversalDetector(NS_FILTER_ALL)
 			char buf[512];
 			fp->read(buf, 512);
 			size_t bytes = (size_t)fp->gcount();
-			HandleData(buf, bytes);
+			HandleData(buf, (PRUint32)bytes);
 		}
 	}