Only include base64.h in tiktoken.cpp

Because it is only been used inside `tiktoken.cpp`.

Differential Revision: [D59664316](https://our.internmc.facebook.com/intern/diff/D59664316/)

[ghstack-poisoned]

Author: helunwencser

examples/models/llama2/tokenizer/tiktoken.cpp

@@ -25,6 +25,7 @@
    limitations under the License.
  *************************************************************************/
 
+#include <executorch/examples/models/llama2/tokenizer/base64.h>
 #include <executorch/examples/models/llama2/tokenizer/tiktoken.h>
 #include <cctype>
 #include <cstdint>

examples/models/llama2/tokenizer/tiktoken.h

@@ -8,7 +8,6 @@
 
 #pragma once
 
-#include <executorch/examples/models/llama2/tokenizer/base64.h>
 #include <executorch/examples/models/llama2/tokenizer/tokenizer.h>
 #include <re2/re2.h>
 #include <cstdint>