st.c: fix comparison between signed and unsigned

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65625 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

st.c

@@ -2036,7 +2036,7 @@ int
 st_locale_insensitive_strncasecmp(const char *s1, const char *s2, size_t n)
 {
     char c1, c2;
-    int i;
+    size_t i;
 
     for (i = 0; i < n; i++) {
         c1 = *s1++;