Only include base64.h in tiktoken.cpp (#4234)

Summary:
Pull Request resolved: #4234

Because it is only been used inside `tiktoken.cpp`.
ghstack-source-id: 233769842

Reviewed By: dbort

Differential Revision: D59664316

fbshipit-source-id: 10168b5cac37f0642f106e5d33cf94857af3a7b3

Author: Lunwen He

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>