src: make base64.h self-contained

This commit includes stdint.h (for uint8_t, uint32_t, and int8_t) to
make it self-contained.

PR-URL: https://github.com/nodejs/node/pull/17177


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>