Bug 1031982 - Cast away -Wsign-compare warning in XML parser. r=peterv

parser/expat/lib/xmlparse.c

@@ -137,7 +137,7 @@ typedef struct {
 #define INIT_ATTS_SIZE 16
 #define INIT_ATTS_VERSION 0xFFFFFFFF
 /* BEGIN MOZILLA CHANGE (Avoid slop in poolGrow() allocations) */
-#define INIT_BLOCK_SIZE (1024 - (offsetof(BLOCK, s) / sizeof(XML_Char)))
+#define INIT_BLOCK_SIZE ((int)(1024 - (offsetof(BLOCK, s) / sizeof(XML_Char))))
 /* END MOZILLA CHANGE */
 #define INIT_BUFFER_SIZE 1024