su_string.c: removed VC signedness warning

darcs-hash:20090109152945-db55f-04727207af3451ace1c339031de003cbf6f77129.gz

libsofia-sip-ua/su/su_string.c

@@ -56,7 +56,7 @@ su_strcasestr(const char *haystack,
   if ('A' <= lcn && lcn <= 'Z')
     lcn += 'a' - 'A';
   else if ('a' <= ucn && ucn <= 'z')
-    ucn += 'A' - 'a';
+    ucn -= 'a' - 'Z';
 
   if (lcn == 0)
     return (char *)haystack;