Fix MSVC6 warnings. (spotted by Tim Peters)
diff --git a/Modules/cjkcodecs/_gbk.c b/Modules/cjkcodecs/_gbk.c
index b5d058a..246b5c6 100644
--- a/Modules/cjkcodecs/_gbk.c
+++ b/Modules/cjkcodecs/_gbk.c
@@ -19,7 +19,7 @@
         DBCHAR      code;
 
         if (c < 0x80) {
-            WRITE1(c)
+            WRITE1((unsigned char)c)
             NEXT(1, 1)
             continue;
         }