hash.c, symbol.c, string.c, mruby-string-ext: undef lesser macro

Add #undef lesser after last usage to prevent macro redefinition
warnings when files are amalgamated into a single translation unit.

Co-authored-by: Claude <noreply@anthropic.com>

src/string.c

@@ -1257,6 +1257,7 @@ mrb_str_cmp(mrb_state *mrb, mrb_value str1, mrb_value str2)
   if (retval > 0) return 1;
   return -1;
 }
+#undef lesser
 
 /* 15.2.10.5.3  */