add undeclared variables

Why did they even exist?

string.c

@@ -6897,6 +6897,7 @@ upcase_single(VALUE str)
 
     while (s < send) {
         unsigned int c = *(unsigned char*)s;
+        const rb_encoding *const enc = 0;
 
         if (rb_enc_isascii(c, enc) && 'a' <= c && c <= 'z') {
             *s = 'A' + (c - 'a');
@@ -6987,6 +6988,7 @@ downcase_single(VALUE str)
 
     while (s < send) {
         unsigned int c = *(unsigned char*)s;
+        const rb_encoding *const enc = 0;
 
         if (rb_enc_isascii(c, enc) && 'A' <= c && c <= 'Z') {
             *s = 'a' + (c - 'A');